Jeff- I think you hit the nail on the head regarding portion size:

My only guess is that larger portions allow restaurants to charge more and make bigger profits (economy of scale: since the preparation cost is the same, you can probably make a bigger profit for a larger piece of meat, pasta, or whatever).

From a marketing standpoint, restaurants don't want a reputation of providing stingy portions (even if it's a minority opinion). Have you ever heard someone say, "I'm never going back to that restaurant, they give you too much food for your money"?

And how many Americans were raised to "clean your plate" when you eat? I think those two factors (large portion size and guilt over wasted food) are largely responsible for our level of obesity. Oh, and maybe the types of food we are conditioned to eat has something to do with it, too.

--Dan.