do you mean it makes your stomach look bigger when you are wearing it or when you take it off your stomach is bigger than it was before?
I do'nt think they do make your stomach look bigger in clothes, although down to underwear, it often draws the eye to the problem area.
I don't think bigger after wear, although it could be you don't do the "not eating much before a big night out" thing to get your stomach flatter for a tight fitting dress, so bit by bit, it could lead you to be bigger.