We've also known for a very long time about a gene in rats which causes them to produce the same hormones as a starving rat even when eating adequate calories, causing them to overeat. Rats without this gene self-regulate their calorie intake when free-fed kibbles. They're living proof of how not every member of a species is identical in how their body handles energy intake and expenditure. If you reduce the calories these rats are able to consume, they become greatly distressed, just like how some humans do when dieting.

Twin studies also prove the heritability of BMI.