Generally speaking though. Those that under eat intentially will almost certainly have a mental issue.I find it interesting that the thin malnourished get pity and understanding. No-one feels the need to abuse them or tell them to just eat a Mars bar. On the other hand there's cart-blanche to abuse the large malnourished. It's their fault. They're weak. They get standard response of "just eat less".
On both extremes, by the time you get to life limiting levels it's a mental issue as much as a physical one.
(And for the sake of full disclosure - I'm definitely one of the big ones)
Whilst some big people may do. Lots will be pure laziness.