Look at the root of the word “fat” and what it truly means. Recognize that all of the negative meanings tied to that word have been fabricated, constructed, and enforced by a society that is severely misguided and misinformed about fatness and health.
Make words your own, make “fat” your own. Talk about it happily. Draw the word on your tummy, look at yourself in the mirror and smile. It is just a word to describe your body size, an adjective. A noun. What it implies is up to how it is used.
If other people have the power to tie negative meanings to otherwise neutral words, then you have the power to tie positive meanings to them.