Quote from: pennywise on January 14, 2016, 10:47:57 PM

Hi,

I have a question.

Am I correct to assume that miner operators can in this moment change the size of the blocks that their nodes produce by altering a certain constant somewhere in the core app?

Yes.

Quote from: pennywise on January 14, 2016, 10:47:57 PM

So what would happen if say 51% of nodes would switch to 2MB blocks? What would be the consequences?

Nothing would happen if 51% of the nodes switched to support 2MB blocks.

However the problems happen when 51% of the miners decide to start mining blocks larger than 1MB and 51% of the nodes supported blocks up to 2MB, then the blockchain would fork, specifically a hard fork. There would then be two blockchains. One blockchain would include blocks larger than 1MB while the other would not. The blockchain with larger blocks would be rejected by nodes that don't support larger blocks. If the blockchain with the smaller blocks happened to become longer than the blockchain with larger blocks, it could orphan that entire other blockchain. However, if the other larger block blockchain was longer than the smaller block one, then two blockchains would continue to persist until everyone agreed to use one or the other blockchain.

The consequences of having two blockchains means that the security of the blockchains would decrease due to each one having less hash power. Confirmation times on both blockchains would become longer as there is less hash power and thus less blocks on each blockchain will be produced in the same amount of time as before. Confirmations wouldn't be entirely reliable as the smaller block blockchain could at any moment overtake the larger block blockchain and orphan all of those blocks.

The problem with this is primarily psychological as many people will probably lose confidence in Bitcoin. It is also a major pain in the ass to fix, as we have experienced similar forking events in the past.

Because of the issues that can be caused by competing blockchains, the block size limit will only change with consensus. It only happens if nearly all of the miners and users agree to support those larger blocks and agree to upgrade to code that supports those blocks. Without consensus we would have competing blockchains which could be disastrous.