What you call 'American pizza' is basically just Italian focaccia with just more stuff on top, with very thick bland crust. Literally nobody in Europe eats pizza in such a form, not in restaurants at least, and for Italians living in Italy that would be a proper insult. Then in my experience there is simply too much on top of it, especially too much cheese, so 1/2 of US pizza is way more filling and has tons of calories compared to full one down here.
I am not commenting on taste which may end up more than fine, just looks. But that thick crust adds tons of empty sugar calories to the food that originally doesn't have so much.