There's no fundamental right for a corporation (which, to remind, is a separate legal entity) to be of a certain size. This doesn't have anything to do with freedom of speech. And if you trim them down to size by splitting them, the original owners would still own the fragments, so it's not a seizure, either.

In America and in many other countries, the mere fact of monopolizing a market is in fact a crime. The constitution offers no protection against this whatsoever. Go google antitrust law, and read up on some history. The Sherman Act, Standard Oil and AT&T are some good places to start.