How is it that some people get fatter with a gastric bypass?

I’ve recently met up with a friend I hadn’t seen in years. He’s always been morbidly obese as long as I’ve known him. About 4 years ago, he had a gastric bypass. When I met him he not only didn’t lose any weight, he GAINED it. I’d say he’s gained about 75 pounds since I last saw him 5 years ago—pre bypass.

How the heck did that happen? I thought you couldn’t eat so much and had to lose weight.