Even guys who can act decently are asked to loose size. The Rock has had to loose size for plenty of roles, even action roles. The general audiences today don't like overly muscular stars. Heck, the fact that most guys want to look like "Brad Pitt in fight club", and think he's big ought to tell you something.
Personally, I couldn't care less. As long as the story/acting/cinematography are up to par, the movie should be great.