Now obviously beer has calories, so a huge intake will contribute to weight gain (especially since you tend to take very little exercise when you're constantly bombed). But even then, it's nothing more than what, say, a strict bacon sandwich diet would do to you -- any excess calories can lead to weight gain. And that weight may or may not settle right on your belly, depending on whether you're genetically predisposed to it.
That's right: There's a beer belly gene. People get fat in different ways, and abdominal obesity is just one of the many interesting fat-storing shapes that the human body can sculpt itself into if said human body doesn't take care of itself. So if you have the gene, you'll wind up having a pot belly eventually, regardless of your actual alcohol consumption. Unless, that is, you maintain a strict diet and exercise regimen for your entire life, but who the hell does that?
"Sixty more crunches and I can drink half a light beer!"
But if that's the case, then where did this fictional correlation between big bellies and beer drinking come from? One possible culprit is cirrhosis, a liver disease of chronic alcoholics that involves the swelling of the abdomen into that familiar beach ball shape. We guess somebody decided that calling it a "beer gut" instead of "organ failure" was less of a buzzkill at family reunions.