It depends on how much it sticks with you. When I was little, I was made fun of for being so skinny. In my mind, I'll always be skinny, although I've been lifting heavy weights and have gained enough to be a "normal" BMI. But to me, I'll always be that skinny girl. I just don't care about it now, and don't apologize to anyone for my body type.
He honestly just might not want the trouble being photographed in a non-Superman state and having it be talked about. Tabloids always expect actors and models to be at their fighting weight all the time, which is just impossible.