i believe there are significant incentives for miner not to create blocks that are TOO big
The relay limitations of all miners are not equal. Orphan risk is unequally distributed based on these limitations, which threatens the viability of groups of miners controlling a minority of hash power = miner centralization risk. See Pieter Wuille's simulation: https://www.reddit.com/r/bitcoin_devlist/comments/3bsvm9/mining_centralization_pressure_from_nonuniform/
What defines "too big?" Without some hard limit that defines this, there is nothing to prevent miners from forking networks.
I see some have little faith in free market here. If miner create too big block, he will find it out after the block become orphaned. So rational miner should be conservative with his block size when there is not hard limit.
So what prevent miners from forking networks? The same as today, risk of their blocks orphaned to other longest chain. No need for some hard limit
That's fine. Nobody forces the majority to do anything. They can fork off onto an alternative network (altcoin) if they want.
Sorry to dissapoint you, but the majority will call longest chain as Bitcoin, and the forkers with smaller chain can call their pet project as they want.