Corrected entry: Jim Belushi impersonates Charles Grodin after stealing his identity. During the film he wears his clothes, including a business suit which somehow fits perfectly despite the fact that they have completely different body sizes.
Corrected entry: At the end of the film, Jim Belushi pretends to be his mother in order to fool the warden and get into the prison during the hostage crisis. After he enters the prison he changes clothes and reemerges as himself in order to be released. So what happened to the mother? Didn't anyone notice that she never came back out?
Correction: The warden does seem to be suspicious and quietly asks Jim where his mother is, but the media is focused on the fact that Jimmy was just released. The Warden doesn't accuse Jimmy of anything out loud to the media because he would look bad making accusations at Jimmy after he was just released.
Correction: Actually the only clothes Jimmy wears of Spencer's are his tennis togs. The suit that Jimmy selects from the closet belongs to Walter Bentley. Since we only see Walter lying in a hospital bed, we can't determine whether he and Jimmy are the same size.