if miners use an incorrect version (generate 8Mb block for example) of the main base of the Bitcoin Network, they loose blocks.

simple.

they can start a new blockchain, if they want ... that's why Exchange have emit a strong signal ...
The miners are not the medium of the network, they run nodes ... but they are not the majority (for now).
They "find" block and ask to the majority if it's a valid block.
For a valid Block, they must wait 100 confirmations of the majority.

That's why full nodes are always a good choice to partipate to the debate.

If 80% of the miners (of the miners group !) emit a block that 80% of the (full) nodes reject -over 1Mb limit- ... and if the 80% of the miners group represent only a 15% of the nodes (the Bitcoin Network), so it's not a fork.

It's a test.