That prevents you from trading with certain people.

Example: I want to sell my labor to B for a salary, but due to corporate ownership being banned I am not allowed to. If you just limit the size of corporations, that will still happen when B has grown a bit and is unable to hire more people due to having hit the cap.

What if we transfer the company to some other entity? Well, I want to sell my labor to B, not to that new entity. Maybe I'm friends with B, or maybe I believe in B's vision, maybe B is giving me joint ownership of the corporation, or some other reason, doesn't matter why it still is a legal limitation.