First of all both women in the picture have no class or style showing off their private parts like that. When you're going out make sure your pants are the right size!
Since we're talking sizes, the average American is more overweight than the average European, but I could find literally millions of pictures to support a claim that there many fit Americans and fat Europeans.
Why are they fatter? I'd blame the fizzy drinks and snacks. The daily routine of many Americans is full of coca cola, sweet drinks from starfucks, muffins, donuts and so on, usually consumed on the go or at work.