By default, our entire language is set up in a way that categorizes and compares things, and then (usually) labels one of them ‘good’ or ‘bad’. No one would have a strong sense of being a woman if there wasn’t an opposite gender to compare to.
Unfortunately, the average person lacks adequate insight into their own feelings to express them clearly and thoroughly. Then we try, but often do a piss-poor job, and we get things like “I feel fat.” It would be impossible to generalize what everyone means when they say something like that. But we know that people with eating disorders, along with being preoccupied with food, often compare themselves to other people. We also know that being underweight and/or malnourished disrupts essential biological processes and brain functions, having a drastic and serious effect on emotional regulation, judgment, and self-perception. To the anorexic mind, ‘fat’ is one of the worst things a person can be, so when all that negative affect is at it’s worse, it’s no wonder it might get generalized into a sense of “feeling” fat.
So, if you find yourself thinking or saying “I feel fat,” I challenge you to try again, and express how you’re feeling without using the word ‘fat’ (or any other descriptors for body size, for that matter!) Maybe you really mean you have a ‘sense of being’ fat. But what about that is significant? What feelings and emotions are provoking that specific idea?