Shorts should fit like a second skin.

They're too tight if when you sit on the saddle, the chamois is pulled so tight that your body can't form to it. Depending on how you're built, it can be possible to wear shorts that are too small. Tummy doesn't have much to do with it one way or the other - but it can definitely be a matter of personal comfort in different brands of shorts. Chamois fit and movement is everything.