I never had a fat girlfriend, but one time, I was hitchiking across the continent, from Washington state to Georgia.
A gal stopped to pick me up in Nebraska. Now that is very unusual for a solo female to pick up a male hitchiker.
One thing led to another, we went back to her apartment in Grand Island Nebraska and had a wild night. Very horny girl.

She was about 30 pounds overweight, when I was in college and had pickings of all those coeds I would not have given this chubby gal a second look.
But she was pretty good company that night in Grand Island.