you have to drink like a hog to gain weight from drinking. but yea. 13 beers is over 1000 calories... the "ultra-light" beer has like 10 fewer calories than miller light. It is all marketing, like javaguru said.
empty calories. if you consume 10 beers that is an extra 1000+ calories your body has to deal with that it cant use. I'm pretty sure it converts it to somethin else and stores it somewhere, but dont quote me on that until one of the chemical guru's explains it in nerd terms