I think just like humans, some cats are predisposed to chunkiness and through domestication, fewer opportunities to burn off their calories.
My home’s an interesting test bed, in a way. I have three cats: a momcat and her two kids. Mina, the mom, is an orange tabby with some Abyssinian in her I believe (face shape is pointy); when I adopted her she’d given birth about three weeks prior, and back then she was shaped like a little pear, with a firm pot belly. (Actually her shape was so weird that I was afraid she had FIP, which can cause a distended belly. The vets confirmed that she was just porky.)
It’s eight years later now, and now Mina’s a 13-pounder still shaped like a pear, much softer but still with a base-belly that’s tight as a drum. Her kids Sophie and Therblig are totally different: Sophie is practically a miniature cat, perfect weight (not to skinny, not fat at all) while Therblig (a boy) is long-legged and lean, sometimes tending to get a little underweight because he’s so damn finicky and hyperactive.
And yet, despite their incredibly different shapes, they all eat the same food (five or six cans a day for all three of 'em). I wish I could get Mina on a diet, but I live in a studio and can’t feed 'em separately, and I don’t want the other normal-sized cats not eating enough.
So what’s the difference between them? I could be generous and say that Mina simply never lost her pregnancy weight but the truth is that she, unlike her kids, overeats – sometimes I walk in and I find her noshing away, and she’ll still be parked there ten minutes later. When I notice that I’ll shoo her away from the dishes, of course, but I don’t want to take the food away entirely since the other kids get hungry.
Anyway, there’s my anecdote (which I know isn’t data). Three cats, all related, all eating the same type of food fed at the same time, living in the same square footage, with the same toys/play opportunities. (Well, Therblig tends to chase his mom and sis around the apartment, so he probably gets the most exercise of all of 'em.) But all of 'em have different figures and eating habits.
Cats are idiosyncratic and have different personalities and constitutions, just like humans. I don’t see why their tendencies to become overweight wouldn’t vary just the same way.