Binge eating is pretty much the only explanation. I can explain with math. Like let's say you take a woman that is 5'6" and eats a bit too much and gets to be like 200 lbs, BMI of 32.3. We'll call this your average american smallfat. For a sedentary person to maintain that weight, it would take about 2000 calories a day, which is pretty easy to hit if they just eat whatever they want and don't give a fuck and sit on their ass all day (and eats out a lot.) A person like that probably snacks too much and eats large portions and calorically dense foods, but probably doesn't go and eat entire packages of oreos in one sitting on a regular basis. If the smallfat took up binge eating and wanted to eventually bloat up to 450 lbs, they'd have to eat over 3300 calories a day. So this would involve eating all the crap your average american smallfat eats, PLUS binging on an entire pizza and/or tray of brownies, etc. every single day.