Your domesticated fat cat could probably use some exercise instead of a vegan diet.

I thought cat was obligate carnivore?

I wonder if it's the standard "if someone has to actually think about the diet, they tend to have a more balanced calorie consumption" thing, where any restrictive diet requires additinal thought. Or were the vegan fed cats being raised by vegan owners? Cause then it could just be that they care more about their pets health since animal wellbeing is part of veganism.

I have doubts that the meatlessness of the food is the major driving factor (statistically insignificant) weight difference