[nikolaj]

Admittedly this is only partially informed, but I am pretty sure most animals don't have access to enough calories to be obese. And those that do (e.g. pets, zoo animals) are (sometimes) kept from obesity due to careful calorie counting by their human caretakers.

[SketchySeaBeast]

I think every labrador retriever on the planet would be 200 lbs if they had their way.

[trevyn]

They also wouldn't be confined to a house or yard.

[briancleland]

Eating is naturally regulated by appetite. That's why appetite and satiation exist. It's difficult to be obese when you eat only unprocessed foods. If you don't believe me, try it out. You might be surprised.

[SketchySeaBeast]

You didn't address their point that zoo animals have to sometimes be kept on diets to remain slim, in spite of their all natural diet. This article seems to think it's a whole gamut of things that keeps these animals thin. http://sciencenordic.com/can-wild-animals-become-overweight

Appetite and satiation are far more complex than just "eat until your full and then stop", for instance amensiac patients have been observed to eat waaay more than they should have just because they did remember eating (Rozin et al. (1998)), even they should have by all rights been physically full. There's whole layers of cognitive, emotional, and behavioral patterns with excess food consumption.

Personally, I have been able to effectively lose and keep off weight while still enjoying all manner of unnatural foods (Ice cream, McDonald's and candy, oh my).