i imagine the 'health at any size' people agree that an overweight pet that is still medically and physically apt is all right. unlike us, pets generally have a smaller margin between a good weight and obesity, and their happiness depends much more on physical activities than does ours - as soon as a pet exceeds a certain weight, either it stops being all that healthy, or the additional weight encumbers some of its physical activities and therefore its overall happiness. a dog that derives most of its happiness from guarding its territory or playing with the master won't be happy anymore once it's too fat to properly run around.

i feel you, anon. i have a pet with allergies, so i can't avoid getting him hypoallergenic dry food, but i sure as hell wish i could.