I don't like the phrase "thin person trapped in a fat person's body" because I think when some people use it, it's a weird form of denial. It's like they think some people are "naturally" fat--with the implication that it is some how a character flaw--but that they aren't one of "those people", they just look like they are.
I know a man who developed an addiction to crack cocaine in his 50s. Over the course of a couple years he went from being on the brink of an early retirement to being hundreds of thousands of dollars in debt. One of the significant roadblocks in his recovery was his refusal to go to NA meetings or anything like that, because those sorts of things were for crackheads. He wasn't a crackhead, he was just addicted to crack. I mean, a crackhead is irredeemable scum, a bad person. He wasn't a bad person, so he couldn't be a crackhead, so all the advice they gave crackheads didn't apply to him.
In the same way, I think people who are ashamed of being fat, who really, deep down, agree with the idea that fat means lazy and weak-willed, will somehow redefine their own fat as an aberration, as something that just happened to them, not something they are. From this perspective it's easy to ignore all the good advice out there as being for "fat people", so you get things like "FAT people need total lifestyle changes, but I just need to lose this weight and go back to normal" (Because fat people must have a bad, stupid lazy lifestyle to be fat, but I am not one of them) or "Fat people need to count calories carefully, but I can just eat until I am full and stop" (because fat people have no self control, but I must, because "normal" people do).
I don't think everyone who feels like thin person "inside" engages in this self-destructive thought pattern, but I think it happens.