The main reason many Americans weigh substantially more than people in other countries, or Americans themselves did thirty years ago, is the combination of corn syrup and variations of sugar in everything, which just doesn't happen overseas where there's far more food regulation on health grounds, and soda as being the norm for drink, instead of an occasional treat. Add that up day after day and it mounts up fast.
Also corporations have convinced people that they need portion sizes far larger than they actually do, I've never heard a fast food restaurant say 'are you sure you need all that food?', but I've had them plenty of times say 'would you like to go large?'.