It's the big gulp theory. The basic laws of restaurant economics state that meals keep getting bigger because food is cheap and fixed overhead--staff, rent, equipment--is the same no matter how much is piled on your plate. So giant servings are a win-win: you pay a little extra for a lot more food, and the restaurant makes extra profit. It's the same rule that created tubs of movie popcorn, venti-size coffee cups and Burger King's Meat'normous Omelet Sandwich. It's why no restaurant will ever give you a reasonably sized stack of pancakes.
So it's surprising that T.G.I. Friday's and the Cheesecake...