I agree, the same seems to hold true for humans and I would imagine other animals as well. Studies suggest that a calorie restricted diet in humans can lead to better health and a longer life.Dogs who look like 'dogs' rather than ??? tend to be much healthier generally. BUT the canine genome is pretty tight overall, and individual breeds are even more inbred, so genetic issues come up way more often. And selecting for traits not compatible with health, as pushed in faces, bulging eyes, dwarfism, and giantism, all unhelpful.
And difficulties that turn up in middle age, like heart problems, for example, are harder to select against in breeding programs.
And mutts often have the same problems as their parents!
Neutering early is now considered a problem in some breeds, and I'm not totally convinced that the very common issue of obesity/ overweight isn't more of a factor here too.
Years ago Purina did a lifetime study of litters of Labradors, with half of several litters fed differently.
Half were kept thin but NOT malnourished, and the others were closer to full fed.
The thin dogs lived two years longer, and had (as I remember) 90% fewer joint problems over their lifetimes.
Probably this study can still be found online... I read it in a veterinary journal.
Mary
https://newscenter.purina.com/LifeSpanStudy
https://www.nia.nih.gov/news/calorie-restriction-and-fasting-diets-what-do-we-know