IPA: /ˈfætər/
KK: /ˈfæɾɚ/
Having a larger body size or more body fat than someone or something else.
After the holidays, I noticed I felt a bit fatter than before.
Fatter is derived from the adjective "fat" (from Old English *fǣtt*, meaning having a large amount of body fat) and the comparative suffix "-er" (meaning more). The word "fatter" means having more fat than another or being more overweight in comparison.
Think of 'fat' as having a lot of body fat, and the '-er' indicates more of that quality. So, fatter means having more fat.