2 fitting parts, 2 different sizes


So i printed these 2 parts, they are supposed to fit together but they printed different sizes. I have printed these parts before and the larger(top piece, the body) does have the chance of printing either as a close fit or a bit larger as shown, how do i avoid this?


Is this a dumb question(did i miss a tutorial somewhere), or does no one have a clue(just like my last problem :P).