A recent study conducted by Turkey's Erciyes University (which hopefully involved timing couples going to Humptown with a stopwatch while cheering them on/taking bets), there's a direct correlation between physical size and endurance. Did you hear that? *jigging belly* Come get some, I may be chubby but I can least last longer than *falls asleep*
The reason behind this is because overweight men have a higher presence of estradiol, a female sex hormone, which actually inhibits the male orgasm.
Apparently the average man climaxes after six minutes, while an obese man can take up to 7.5, neither of which is anything your girlfriend is going to write home about. Mostly because people don't discuss their sex lives with their parents. She'll definitely tell some of her girlfriends about it though, then you'll always wonder when they're around if they know you're a sprinter and not a distance runner. And keep in mind this is for obese men, not morbidly obese men, who may never finish and will die on top of you.
Thanks to Thaylor H, who agrees trying to argue that you only last a minute because you're at peak physical fitness is an uphill battle.