Exactly. Large companies love regulations when they affect everybody since they can easily abide the regulations while they help to destroy potential competitors.
Keeping regulations focused on big players serves the dual purpose of focusing regulation where its affect will be most significant, while also ensuring it doesn't negatively affect the market. But yeah, like you're mentioning the big problem is that once companies reach a certain size they begin to develop the political connections necessary for them to simply kill, or at least castrate, any potential regulation that might genuinely require them to behave in a way that is inconvenient - even if it's better for society.
The ideal of some arbitrary cut off point has been tried in lots of scenarios, and is gamed by all parties.
Example: Copyright will protect new works for x years, at which point Disney lobbies for the arbitrary goal posts to be moved.