Why are Kakapo fat?

Kakapo are not fat, but they are the world's heaviest parrot. This is because they are ground-dwellers, and the world's only flightless parrots. Their legs need to be stronger, as they spend their time climbing rather than flying, so they have solid leg bones and more well-developed muscles in their legs, while flighted birds have well-developed chest muscles to enable them to fly.