I actually think this is a good idea. Surely you can have a good trainer who is in sore shape but more often than not you want to learn from someone who can talk the talk and walk the walk. Someone who is clearly in good shape is someone who's opinion you'll have respect for. Is anyone who doesn't have a clue about fitness really gonna listen to a fat guy whether he knows his shit or not? They typically want abs, they don't know how to get them, so wether fat guy tells them correctly or not they don't know and all they see is a fat guy.