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The (2015–2017) was the most contentious fight in Bitcoin's history — a battle over Bitcoin's soul. On one side: “Big Blockers” who wanted larger blocks so Bitcoin could process more transactions and serve more users. On the other: “Small Blockers” who insisted that keeping blocks small was essential to keeping Bitcoin decentralized — and that scaling should happen on Layer 2 (Lightning) instead. The war ended with SegWit activation, a chain split (Bitcoin Cash), and a permanent lesson about the limits of on-chain governance.
The issue: 1 megabyte
Bitcoin blocks are limited to 1 MB (megabyte) of data. Each transaction takes up some of that space, so a 1 MB block holds roughly 2,000–3,000 transactions — about 7 transactions per second (tps). For comparison, Visa processes ~24,000 tps at peak. As Bitcoin grew in 2013–2015, blocks began filling up, and transaction fees rose. Users faced a choice: pay higher fees or wait longer for confirmation.
The 1 MB limit was added by Satoshi in 2010 as a temporary anti-spam measure. At the time, blocks were tiny (under 100 KB), and the limit was uncontroversial. Satoshi never explained the reasoning, and left no guidance on when or how to raise it. After Satoshi disappeared, the limit became a fixed constraint — and changing it required consensus, which was nowhere to be found.
The two camps
Big Blockers
Position: Raise the block size limit (to 2 MB, 8 MB, or unlimited) so Bitcoin can process more transactions, lower fees, and serve more users. Bitcoin should be peer-to-peer electronic cash — a payment system, not just a store of value.
Key figures: Gavin Andresen (Satoshi's chosen successor), Mike Hearn, Roger Ver (“Bitcoin Jesus”), Jihan Wu (Bitmain CEO, manufacturer of mining ASICs).
Argument: If blocks are full and fees are high, Bitcoin loses users and utility. Keeping blocks small makes Bitcoin a settlement layer for the wealthy, not the inclusive payment system it was designed to be. The decentralization concern is overblown — storage and bandwidth keep getting cheaper.
Small Blockers
Position: Keep blocks small (or raise them only minimally) to keep running a full node affordable. Scale Bitcoin on Layer 2 (Lightning Network, sidechains) rather than on the base chain. Bitcoin's base layer should be a secure settlement layer, not a high-throughput payment system.
Key figures: Gregory Maxwell, Pieter Wuille, Adam Back (Blockstream), Luke Dashjr, Matt Corallo.
Argument: If blocks grow to 8 MB or more, running a full node becomes expensive (more storage, bandwidth, CPU). This centralizes validation: only large entities can run nodes, and ordinary users must trust them. A decentralized network of affordable full nodes is Bitcoin's key security property. Sacrifice throughput to preserve it.
The timeline
Tap any event to expand its story.
Satoshi Nakamoto adds a 1 MB block size limit to Bitcoin. The original code had no limit; this was added as a temporary anti-spam measure. Satoshi never explained the reasoning publicly, and the limit was not controversial at the time — blocks were well under 100 KB.
As Bitcoin grows, blocks approach the 1 MB limit. Transaction fees begin to rise, and confirmation times become unpredictable during periods of high demand. Developers and businesses start discussing whether to raise the limit.
Two camps emerge. "Big Blockers" (led by Gavin Andresen, Mike Hearn, and later Roger Ver and Jihan Wu) argue that bigger blocks are needed for Bitcoin to scale and serve more users. "Small Blockers" (led by Core developers Gregory Maxwell, Pieter Wuille, and Adam Back, plus Blockstream) argue that bigger blocks would centralize mining (fewer people can run full nodes) and that scaling should happen off-chain (Lightning Network).
In a closed-door meeting in Hong Kong, Bitcoin Core developers and miners agree to a compromise: activate SegWit (which effectively increases capacity) and then implement a 2 MB hard fork. The agreement collapses when Core developers later back away from the hard-fork commitment, arguing it was never an official Core position.
A grassroots movement of users and businesses proposes BIP 148, a User-Activated Soft Fork: activate SegWit by August 1, 2017, or reject any blocks that don't signal support. This is a threat to miners: comply or your blocks will be ignored. The UASF movement, led by developer shaolinfry, shifts power from miners to users.
Facing the UASF deadline, miners signal support for SegWit. SegWit activates on August 23, 2017, effectively increasing block capacity (by moving signature data out of the base block) and fixing transaction malleability (enabling Lightning). The Small Blockers win the SegWit battle.
On August 1, 2017 — before SegWit activated — the Big Blockers execute a hard fork, creating Bitcoin Cash (BCH) with an 8 MB block size. BCH inherits Bitcoin's history up to the fork point but goes its own way with bigger blocks. The split is the final rupture of the two camps.
A follow-up compromise (the "NYA" or SegWit2x) proposes a 2 MB hard fork after SegWit. It collapses in November 2017 when the signatories fail to agree. Mike Hearn had already declared "Bitcoin has failed" and left the project in January 2016. Bitcoin Cash itself later forks into Bitcoin SV (Craig Wright) and Bitcoin ABC, fragmenting the Big Block side further.
SegWit: the technical compromise
(Segregated Witness) was the Small Blockers' key technical tool. Rather than raising the 1 MB block size directly (a hard fork), SegWit moved signature data (“witness data”) out of the base transaction block into a separate data structure. This effectively increased block capacity to ~2–4 MB (depending on transaction mix) without a hard fork — it was a soft fork, backward-compatible with old nodes.
SegWit also fixed transaction malleability — a bug that had caused real problems for protocols that relied on transaction IDs. (Mt. Gox famously blamed malleability for its collapse, but that claim was later discredited: researchers found the bug could account for only a tiny fraction of the exchange's missing ~850,000 BTC, which had been lost over years regardless.) Fixing malleability was a prerequisite for the Lightning Network, a Layer 2 payment channel system that could scale Bitcoin to millions of transactions per second off-chain. For Small Blockers, SegWit + Lightning was the answer: keep the base layer small and secure, scale on top.
The UASF: users vs. miners
By early 2017, SegWit had been ready for months, but miners — many aligned with the Big Blockers — refused to activate it. The Bitcoin improvement proposal BIP 9 required 95% of miners to signal support, and they were stalling.
The response was BIP 148, a . Instead of waiting for miners, BIP 148 said: starting August 1, 2017, users (exchanges, wallets, node operators) would reject any block that didn't signal support for SegWit. If miners didn't comply, their blocks would be ignored — and their block rewards worthless. This was a direct power play: users, not miners, decide the rules.
The UASF threat worked. Facing the prospect of mining worthless blocks, miners signaled support for SegWit. It activated on August 23, 2017. The Small Blockers won — but the Big Blockers weren't done.
Bitcoin Cash: the split
On August 1, 2017 — three weeks before SegWit activated — the Big Blockers executed their own hard fork: Bitcoin Cash (BCH), with an 8 MB block size and no SegWit. BCH inherited Bitcoin's transaction history up to the fork point, then went its own way. Anyone who held Bitcoin at the fork moment received an equal amount of BCH.
The split was the final rupture. The two camps, unable to agree, went their separate ways. Bitcoin (BTC) pursued the Small Block path: SegWit, Lightning, and a small, secure base layer. Bitcoin Cash (BCH) pursued the Big Block path: larger blocks, on-chain scaling, and a payment-system vision.
Bitcoin Cash itself later split again, into Bitcoin SV (Craig Wright's faction, which wanted 128 MB+ blocks) and Bitcoin ABC. The Big Block side fragmented, while Bitcoin (BTC) consolidated around the Small Block vision.
In terms of market outcome, the Small Blockers won decisively. Bitcoin (BTC) is worth vastly more than Bitcoin Cash (BCH), and Lightning Network has grown into a functional (if still maturing) Layer 2. But the Big Blockers' concern — that high fees price out ordinary users and make Bitcoin a settlement layer for institutions rather than electronic cash for everyone — has proven partially prescient. Bitcoin fees are high during congestion, and most “payments” happen on Lightning or custodial platforms, not on the base chain. The trade-off the Small Blockers chose (decentralization over throughput) is the trade-off Bitcoin lives with today.
The lessons for governance
The block-size war taught the crypto industry several governance lessons:
- Hash power is not governance. Miners can't force rule changes on users — users can reject non-compliant blocks. The UASF proved that economic nodes (exchanges, wallets) have the ultimate say in which chain is “Bitcoin.”
- Forks are the resolution mechanism. When consensus can't be reached, the chain splits. Both sides get what they want — in separate chains. This is messy but peaceful.
- Soft forks are easier than hard forks. SegWit (soft fork) succeeded; the 2x hard fork failed. Backward-compatible changes are easier to adopt because they don't require everyone to upgrade simultaneously.
- On-chain governance has limits. Bitcoin has no formal governance process. Decisions emerge from rough consensus among developers, miners, and users — a slow, messy, but ultimately robust process. The block-size war showed both the strengths (decentralized resistance to capture) and weaknesses (years of drama, wasted effort, and community damage).
Key takeaways
- The block-size war (2015–2017) was a battle over whether to raise Bitcoin's 1 MB block limit. Big Blockers wanted larger blocks for more throughput; Small Blockers wanted small blocks to preserve decentralization and scale on Layer 2 (Lightning).
- The Small Blockers won. SegWit (a soft fork that effectively increased capacity without raising the base limit) activated in August 2017 after a UASF (User-Activated Soft Fork) forced miners to comply.
- The Big Blockers hard-forked into Bitcoin Cash (BCH) on August 1, 2017, with 8 MB blocks. BCH later fragmented further (Bitcoin SV, Bitcoin ABC). Bitcoin (BTC) pursued the Small Block path and remains dominant by market cap.
- The UASF proved that users (exchanges, wallets, node operators) have the ultimate say in which chain is “Bitcoin” — not miners. Hash power is not governance.
- Bitcoin's base layer is a small, secure settlement layer; most payments happen on Lightning or custodial platforms. The trade-off (decentralization over throughput) is the one the Small Blockers chose — and it has proven to be the market's preference.