Why single out one cause, when there's obviously many.
Take food. I live near a supermarket that is probably three times the size of the one my parents went to, but the produce section is smaller, the meat and dairy sections about the same size. The surplus acreage is taken up with cheap, calorie dense, no-preparation convenience food.
Or the fact that Amercians spend more time in cars than they used to, on average over 290 hours a year.
Here's another interesting fact: research shows that the portion size you choose is positively correlated to the size of the package you serve yourself from; this doesn't happen consciously, it's just that a cup of cereal from a 9 ounce box appears like a lot more than a cup of cereal from a 21 oz box.
The huge sizes are driven in part by an attempt to cut down on trips to the grocery store. American home kitchens are the largest in the world, and most of that is needed for storage because we don't do very much food preparation.
So if there's a single root cause it's the pursuit (sometimes failed) of efficiency; we have the wealth to try to reduce labor and time spent doing things, but our bodies are designed to spend time doing things.