You know, this is a question I did NOT expect to get. Originally Pewna was just a R63 version of Pewny, created for an RP in which potions were mis-used. In general it was not a separate character at all. Fast forward a few years later, when I actually used her a bit more for RPs with certain people. I wanted it to make more sense, not just ‘a genderbent char’, which would be especially weird, considering Pewny is straight, and he would be hit on by stallions. So I decided to make Pewna his twin sister, with her own, somewhat different personality and such. 

As for why Pewna is fat? There is two reasons to that:
1. She LIKES to be fat,
2. To please the stallion who introduced her to weight gain