In the US, Congress decides the rules, the FTC and the Department of Justice decide when they believe a corporation is too large, and the court system decides i

In the US, Congress decides the rules, the FTC and the Department of Justice decide when they believe a corporation is too large, and the court system decides if they're right. The corporation can then be stopped from acquiring other businesses, stopped from executing certain business practices, or broken up into smaller companies. Generally a judge ensures that the plan is carried out.