Is this derived from studies or from your experience?Most grazing/browsing animals would very quickly kill themselves given unlimited access to grains and most pet cats and dogs will overeat to obesity given the opportunity.
It hasn't been my experience for my cats, and for the horses I know. But my single experience isn't meaningful.
I don't think there is much food to be found in nature that is as addictive as the stuff humans have invented. Sure, animals can be addicted to sugar, but corn can't be compared to ice cream.
And animals are not submitted to commercials. So I think obesity in humans is as much a sociological problem as a health issue.
Pellets aren't very common for chickens in France, most commercial food are in the form of fine mill. My chickens do pick what they prefer. I have two flocks and the one that free ranges completely leaves out the fine floor, which is basically the soy and supplement, whereas the flock that is contained in a yard tends to eat more of everything although they do pick out wheat and corn pieces first. And they have access 24/24 to their food and they certainly don't spend the whole day eating. But it's just my single experience and I suppose for every single experience someone else will have the opposite.