BinaNeko
I dont even think a fat princess would sell.
Little kids love beauty, and by the sense of beauty, fat might not be beautiful (look at the picture of ariel forexample). However, a chubby princess might work, chubby could be cute, and somebody chubby doesn't necessarily have to have health-issues either! So yeah, a chubby princess might work!
Actually, Dreamworks proved a plumper princess can sell with Princess Fiona in Shrek.
Granted she was not the 'star' of the story but she played a pretty big part.
And they did make Fiona rather pretty even though she was a plump green ogre.
One of the things I like about the Ghibli Studios movies is the heroine is often rather average looking. They tend to be thin but I think that is largely because they are usually young girls. There can be fat kids of course but on the whole children's metabolisms are faster. When I was a kid I could skip desert at lunch and lose 5 pounds a week still otherwise eating all I wanted. Harder to do as an adult. My point is they show children "a kid like them" making the right choices or learning from bad ones rather than some idolized version of a woman most kids will never grow up to be. Plus the Ghibli heroines tend to have brains as well. Some of the Disney lot seem rather vacant. Though I guess Princess Mononoke might be the only Ghibli heroine that is actually a princess.
As to Disney making a fat princess just to champion diversity - just - NO.
My life can be complete without a fat princess and a one legged princess and a princess in a wheelchair etc. In the end though Disney will make what ever movie fodder they believe will sell tickets.