I think obesity dwells in the intersection of disease and lifestyle. It seems pretty clear that some people can "get away" with eating lots of fatty foods, sweets, carbs, etc., more than others. (Heck, I could get away with it a lot better 10 years ago than I can today). I'm not commenting on the long-term wisdom of doing that, but just observing some people can eat that way and not be anywhere close to fat.
At the same time, it's pretty clear people make eating and exercise choices that make a weight gain tendency much worse.
I can't tell you how many times I've been to dinner with a friend, who at one point in the conversation exclaims how remarkable it is I haven't gained a lot of weight (as he or she has), only to order an appetizer, an entree, dessert and alcohol while I order two appetizers or one entree and water. And to a person, they never put it together (at least out loud) that they consumed easily twice the calories and spent three times the money I did.
P.S.: I'm not a paragon of physique healthiness. I've never had a six pack (though there have been times my stomach was almost flat). I have skinny arms, something of a belly, a butt that's starting to droop and chest fat. But at 5-11 and 172, I'm reasonably HWP. It used to come somewhat easily. Now I have to really work at it. I don't think I have it in me to flattened my stomach and muscle up, but I am going to aim in that direction. Right now, I'll be happy with less stomach and chest fat and a little more muscle. I'm an avid walker, but I start to fall apart after that.