I mean, some of it, I think, has to be delusion -- at least for some. It works the other way, too, after all. Before I discovered my inner shitlord, I had a friend near a 26-28 BMI, and I thought for the longest time she and I were the same shape/size, even though my BMI was around 22-21. We stood in front of a full-length mirror once, and I was blown away by how much tinier I looked than her, despite only have an inch difference in height and about ~40 lbs in weight (though that was by her claim to what her weight was -- I'll never really know if she was being honest or not). And I'm sure she looked at me and thought, "We're the same size", too, until then.