I suppose fat can sometimes help insulate for cold temperatures but that can't be the only thing. I'm thin and tolerate low temperatures well, and some fat people don't.

I notice my tolerance goes up and down from winter to winter, and I suppose acclimation is one big factor. My weight doesn't vary much from year to year or season to season.

To acclimate to the cold, I expose myself to it at least a few seconds a day. Also since I want ride as much as possible through the winter, I find it's a good idea to ride at least once a week. Without doing that, missing a week of riding in the cold makes the time I get back on harder.

One winter during Pandemic, my morning ritual included going outside naked (I was living in the woods at the time) and standing there barefoot. Some days it was brutally cold, so I didn't stay out more than a few seconds, but I also didn't incur any pain. Ever since then, my feet tolerate cold much better, even now, a few years later.