A partial explanation is that popular images of Siddhartha Gautama (especially in the West) conflate him with Budai, the fat monk and future Buddha of Chinese Buddhism.
The "Fat Buddha" is not Siddhartha Gautama. He's Budai, for some traditions an emanation of Maitreya, who is the final Buddha yet to come. Maitreya still lives in the "heaven of the contented." The Fat Buddha is an image of a Chinese Monk from C10 CE (who some consider Maitreya's emanation, others an image of a bodhisattva--one on the final stages of awakening with godlike qualities). That probably wasn't the fun answer you were looking for. Religious Studies rarely offers fun answers. ;)
I'm sure someone else can explain the "fat Buddha" phenomenon better than I can. But the skinny Jesus, I think, comes from the modern desire to seem him as physically fit and attractive in accordance with broadly Western concepts of beauty. We don't want a fat Jesus any more than we want an ugly Jesus, apparently (as Colbert reminded us the other night). Eric
Because Jesus was 33 and Buddha was 80! The metabolism slows down, don't ya' know?
That's not the Buddha. That's Budai/Hotei. http://en.wikipedia.org/wiki/BudaiEmaciated depictions of the Buddha are not uncommon in Buddhist art.
You know who else was crazy thin? That Ghandi, yo.And they say the camera adds ten pounds. Pssshaw.