From my Rajan post:
Regulation is ineffective. The profit motive is an irresistible force. Regulation is not an immovable object.
The trick is to use taxation to make different things profitable. Use it to make some things more profitable, and other things less profitable.
For example, the existing business income tax favors bigness. So, if things go their way, a business can grow like crazy. That's how you get giants like Microsoft and Google and the too-big-to-fail banks.
And then we have anti-trust, because sometimes we seem to think some companies are too big. But it would be much more effective just to change the tax system. We could redesign the business tax to favor small business instead, for example. But it would be better to make the tax so it favors no one by size. Let Walmart compete on equal footing with Mom-and-Pop.