Yeah, true. But if you look at that image... if you actually do subtract the overhead, they will be about 800cc in the negative! Meaning there's no way they can keep that many systems.
Correct assumption. That is what happened to Hudson in the 1.3beta phase. But since nobody could see overhead, we did not understand, why CC was negative from one week to another.
Now, I understand, that overhead was either introduced or tuned up between two beta cycles. As a result, several Hudson systems were send into turmoil and all pledges were busy fortifying all systems to repair the damage.
It will not be other players stopping a power expanding, but overhead. If player change powers due to being limited to fortifications every cycle, it will accelerate a powers downfall until systems and overhead is reduced enough to restore a healthy balance.
As to how it is calculated, I do not now exactly, but guess, that is some exponential formula linked to the number of controlled system. Every added system will increase overhead more than the last time. Let us hope, that FD does not manually influence the value too often...