Fat is not a feeling
I often catch my friends commenting that they "feel fat" when they are not at their best emotionally. They "feel fat" when they are having trouble dating or in their relationships. They "feel fat" after they stayed up until 2am racing to meet a work deadline. They "feel fat" after having too many beers and hot wings at a Super Bowl party.
When someone says they "I feel fat", they are expressing a feeling of discomfort projected onto their body shape. But oftentimes, the feeling goes much deeper