I think it has to do with the insane amount of fat foods they eat. Of course, you can eat that stuff in moderation, and I did and only gained 2lbs the whole time (despite drinking a bottle of wine a day). Their unofficial national dish is called a pastechi, though, which is this and filled with high fat cheese and meat. Oh, and they have way more/different junk food (like, for example, reeses cup oreos).