Controlling a Company

In the world of the small corporation, with which I am familiar, you need to own 50.1% of the stock to control the company. The world of the mega-corporation is a lot different. I've seen cases where as little as 7% of a company's stock gives you control. Then, there are the companies with two classes of stock where the percentage needed for control could be even less. Why don't we control AIG? We own 80% of the stock.