When Obama speaks of the "Big Corporations," he is talking about the average American. The average American owns the big corporations through their pension funds, mutual funds, and through the foundations and endowments that they support. There isn't some rich corporation guy out there to go after. If you tax Exxon, then the average American's stock holdings suffer and the average American's future pension payments will be less.
So, what Obama should say is "Let's go after the Big Corporations, so that the average American can retire on less money." The facts are that taxing large corporations is a tax that is almost totally borne by folks who buy their products and own their shares -- that's middle America.
So, if you want Americans to retire with a significantly lower standard of living, then, by all means, tax the heck out of the corporations that they own.