Hi again, mexcelaya!
As alluded to in the post by darkvision, "big" is a matter of perspective and definition.
Your friend in school might not look fat, but perhaps others are correct in saying that he's big. Just saying someone is big doesn't mean that they're fat or even a put-down. It's a matter of perspective and definition and can be very subjective as far as the interpretation as to what is big is concerned.
Why specifically does he feel bad, please?