Actually, beerbellies comes from both. Alcohol, with respect to food and nutrition is a carbohydrate. Unfortunately, that kind of carb has no nutritional value whatsoever, so your body will never really use it as energy like other carbs. Fortunately for people it's pre-worked on by the liver and kidneys and, as quickly as possible, its ejected from the body (thus the immediate dire need to take a leak after a few beers and the reflex of throwing up after you've had too many).
Now, this would all be fine, since most of it leaves the body just as it goes in. Unfortunately though, ingesting a large ammount of carbs, especially simple carbs(sugars), causes your insulin levels to spike sharply. If these carbs come in liquid form, such as beer, they're absorbed almost immediately in the bloodstream.
So drinking any alcohol has the potential to leave you with a "beer belly" but because beer is the drink with the bigger volume which is made specifically out of a large source of carbs (cereal starches), it has a greater effect than say wine. Because it's made out of starches, it also contains some carbs that CAN be used as a nutrient by the human body.
What an insulin spike like the one caused by alcohol does is it speeds up the synthesis of bodily tissue. Everyday a lot of your cells die off and are replaced with others as other cells divide to take their place. Insulin plays a great role in pre-working the nutrients we ingest in order to make them ready for tissue synthesis. What this means is that somewhere in your body, your blood is going to have to deposit all of these construction materials into cells that are multiplying, there's no way around it, it will have to do it. So, in the order of importance, it will first feed your vital organs, but as vital organs have small needs for cell reconstruction, it will then go to your lean body mass (skeleton and skeletal muscles) and once all your needs are fulfilled it will deposit the rest as fatty tissue, doesn't matter where that fat goes, but the fact is that for most people fat deposits most on the belly.
This is the same trick used by bodybuilders to grow in size, since just after a workout they ingest a large quantity of sugar in a liquid form, which causes the body to make an insulin spike and some minutes after that they ingest a large quantity of protein in liquid form. The fact that they worked out before and their muscles are the parts of their body that crave nutrients and reconstruction the most makes them the most likely target for those "construction materials" that will be made. Also, the fact that they are tired after the workout means that the sugar will be ingested from their blood quickly as energy, thus it won't deposit as fat, and because they take in protein, it will be used to construct muscle tissue. But if you haven't worked out prior to ingesting beer, fatty mc fat fat will be visiting you that evening. There's even more reasons for doing this habit, but I won't go into them right now.
So the moral of the story is:
Beer is a large liquid source of simple carbohydrates, most without any nutritional value. And that causes people to get fat, thus beer bellies.
However, any other large liquid source of simple carbs (simple carbs = sugars) would do the same to you. So soft drinks are the most likely thing you ingest that will make you fat (other than saturated and trans fats which will make you fat just because of calorie ammount alone).
The difference is that beer is, sometimes more than others, consumed on its own and it isn't consumed for thirst or hunger, thus it is extra intake; while consuming say a soft drink full of sugar after a meal will have a smaller effect on your blood sugaer levels, since other nutrients are consumed at the same time. If you consume a hefty quantity of fat and protein (meat) about an hour before ingesting beer or soft drinks, it has a smaller chance of adding to your waistline, since other nutrients are in the blood as well and dilute the bloodsugar concentration. However, constantly drinking soft drinks with a lot of sugar for thirst will drastically increase your chances of gaining fat, because you get thirsty more times than you get hungry, thus a lot of times you ingest the sugary drink on its own. Which is a bad idea.
So, if any of our forum members have weight problems, this is your wakeup call: GIVE UP POP, COKE AND OTHER SOFT DRINKS NOW!!! They kill any diet or exercise you do. The only drink that anyone should be drinking for thirst is water. It will hydrate you properly, it will thin your blood so your insulin levels will remain low, .... I can't stress the benefits of drinking more water and there's plenty of them. So, however addictive soft drinks are for you people, remember what I wrote here whenever you pick up a can or bottle of pop and DO NOT DRINK IT. And exchanging beer with wine or liquor as a source of alcohol would be a good idea too (or even better, giving up alcohol altogether, or consuming it very rarely ... say drinking only at parties, one party you drink, the other one you don't ... even better if you do this with buddies, switching around who will be the designated driver).