You searched for: “obesophobia
obesophobia, pocrescophobia (s) (noun) (no plural)
A loathing of gaining weight: It seemed that Linda was always on a diet because she was awfully afraid of becoming fat and not fitting into her slim-fit jeans anymore, and her friends thought she had obesophobia!

Jack, troubled with pocrescophobia, was always sure he had a very slim figure by not eating any cake or ice cream, and went to the fitness studio many times during the week.