Short: body contrast, eating, trying to do XYZ normal human thing and struggling, enabling partners, self delusion. It's a mix.
Long: Gluttony, sloth, entitlement and being physically unfit drive me nuts. When a woman acts dainty, feminine or like a princess/expects that treatment but weighs as much as 3, it's wildly hot to me, especially if they're sort of a dull mouth-breather. I think the contrast of femininity or this sort of hyper-traditional femininity I'm describing with their ill-maintained figure somehow makes them more feminine and appealing. Unfortunately that combination of personality traits is probably the very worst you could ask for in a long-term partner, but this is just fantasy or wank territory for me and similar pleasures along those lines can be found in less exaggeratedly degenerate women.
Fat deposits on the limbs and unwieldy spots like the neck, arms, hands and upper back/shoulder blades. Obviously, the gut too. When a pretty woman gets fat or obese and her shape doesn't follow a sort of 'traditional' contour (e.g., Boberry is something of an example of still having a 'traditional' shape despite her size). I love the way a naturally beautiful face looks when it's swaddled in a thick, bulging donut that wraps around their head and neck. Lazy, weak, obese arms and useless manicured hands good for nothing but reaching for food, a phone or just swinging limply while they waddle. Physical unfitness and the general habitual shortcuts obese women take- Asking someone else to do something so they don't have to move, only reaching or moving their forearms and not the cumbersome, heavy upper arms, leaning and sitting whenever possible, the little poorly-disguised gasps for air in between sentences when they're out of breath, or even just had spoken too much in one go. No matter how body positive they may be, they almost invariably still will try to hide some of the more embarrassing side effects of their weight and feign normalcy in these various situations. The more you look for it, the more you'll see big women (and just fat people generally) making use of this "obese playbook" of sorts, consciously or not, and it's an adorable little show for me every time. Every time a big woman habitually pulls an article of clothing back into her desired position, all of those other things... It makes me want to dick them down, pamper them and enable every last naughty little habit that got them so porky.
I love the idea that women this huge were an impossibility or at least rarity as little as 50 years ago and now they're everywhere. The concept of the obesity epidemic as a whole is something I could nut over.