The main problem is that our culture is so in love with sugar that we have actually started adding huge amounts of it to stuff we don't even eat, namely dog and cat treats. There is absolutely no need for this. I don't know if you've noticed but there is not exactly a lot of natural sugar in the mice, birds and fish that our pets would eat in the wild. By adding ingredients they have never experienced as a species we are actually "rewiring our pets' behavioral responses" and "creating cravings" that have literally never existed in dogs and cats before. We're taking animals that have never had a desire for sweet things and making them beg for peanut butter because it's so adorable when they try to lick it off and then they use their little paws like tiny furry people and...
Ahem, sorry. My point is: Screw you, thousands of years of evolution, I want a 30-pound kitty.
"I can haz heart problems down the line?"
It's actually gotten to the point where the average pet owner sees an overweight animal as "normal." We're so used to seeing super-sized bundles of fur running around (or just sort of laying there having minor heart attacks) that our perspective on what size a dog or cat should be has changed. Forty percent of owners with obese cats, not just overweight but obese, think their cats are at a healthy weight. It's this bizarre change in perspective that affects almost every aspect of the world-fattening (or Chubpocolyps. Or Fat-Assmageddon.)