For me, the large girl often comes with wrinkles or folds in the skin, and sometimes cellulite as well. For me, a smooth complexion, and smooth contoured body is what I prefer. That's the initial unattraction for me.
On a deeper level, if I were to become involved with a larger girl, she wouldn't be able to keep up with my lifestyle. My lifestyle includes mountain biking or road biking around 50 miles a week (between mountain and road together). Or snowboarding in the winter. Or even just lots and lots of walking around (I don't use my car unless I'm pressed for time and need to get somewhere quickly. . . otherwise, I use my bike or walk). She wouldn't physically be able to keep up with me.
I'm not saying that ALL thin girls can either, but the odds of someone thin being able to keep up are certainly greater than the odds of someone large. And so I play the odds when I look for a girl.
Alexi