Portions in North America are HUGE.

The restaraunt portion thing is... a big problem. Here's what I think is going on.

I'm pretty sure that it has more to do with profitability than customer demand, although it's gone on for long enough perhaps it's both by now. The key here is that food sales have pretty thin margins (except for soft drinks which are outrageously marked-up everywhere). If a restaurant chain suddenly downsized their portion sizes, people would realize very quickly that the price hasn't scaled down to the same extent, as the current portion sizes are inflated to mask how much food service really costs. There's a price floor to remain profitable and I think it's a lot higher than people realize.