"genetic correlation"

this probably plays a part in it for some, but only a small part if I would have to guess, there's way more to it.

First sign there's much more going on than just 'a disease' is the fact there are way more obese people in the US for instance than in most other countries. And also for instance that children where both parents are obese are more likely to develop obesity as well. First comes down to culture, second to lifestyle. Not two things I'd directly associate with words like 'genetic or 'disease'.