Having traveled throughout Europe since the 1980s, yes, Europeans in general are not nearly as fat as many Americans, but the gap seems to be closing as American fast food has become more prevalent. Well, American everything has become more prevalent.
Showers can be comically small.
No AC in many hotels and cars can be quite unpleasant. We were fortunate the owner of our VRBO provided a fan, or it would have been difficult to sleep a couple of nights.
Yes, Europeans are much better at conservation and are more environmentally friendly. Again, in general. Europeans tend to dress better than Americans, where around here you see people in pajamas and bedroom slippers in stores and restaurants.
As for portion size in the US, we normally get an entree and a big salad, and split it. Even that is often more than enough. Then again in Germany a jagerschnitzel with pommes and salad is as high calorie as typical American restaurants.
I'd hazard to guess that if reusing towels wasn't profitable, hotels wouldn't do it. The bottom line and profit are the main drivers.