When I say "I feel fat," it doesn't mean "I feel fat." I don't think I am fat, or look obese. Probably no one else notices that I look different, based on my feeling fat.
When I say that I feel fat, it means I feel as though I haven't worked out in a while and have been eating yucky foods and drinking too much and my pants are not fitting well and right at that moment in time, I don't feel as though I am at my best.