Just like how "sit" becomes "sat" in the past tense, "fit" becomes "fat" in this new linguistic evolution.
"These shoes fat me last year."
"These shoes used to fit me last year."
"The dress fat perfectly at my wedding."
"The dress fit perfectly at my wedding."
"When I was younger, this hat fat me well."
"When I was younger, this hat used to fit me well."
This linguistic evolution moves away from the negative connotations of the old meaning of "fat" and repurposes it as a useful past tense form. It's shorter than saying "used to fit" and follows similar patterns to other English verbs (sit→sat, fit→fat).