Mochivation-sama
My name is Tetsuya Urakami from Nagomi-an.
As you said, the statue of Shakyamuni's penance is extremely ugly. It shows the Buddha being ascetic for 6 years without being a monk, and you can even see his spine from his retracted stomach.
http://www.higashino-kobo.com/gallary/kugyouzou.html
However, there is a commandment to only eat in the morning even after stopping penance, so I don't think I was able to get plump.
I'll touch on Amano's answer a little bit, but it was probably a symbol of wealth and health in any country before modern times, and I think a plump one would have a preferred appearance. Even in Japan, beautiful Heian women have a puffy face.
Among the Buddha statues, the Kongo Rikishi are exciting, aren't they? After all, I wonder if people in the old days had a variety of tastes, such as those who liked being plump and those who liked being muscular.