@oristarA,
Hi oristarA,
If you are describing the woman objectively you can say she was a fat or an obese woman. If you are talking about a friend or colleague or family member , you might want to be more diplomatic: heavy, plump, overweight,chubby, chunky, stout.
In old circus sideshows they had "The Fat Lady". ....and we have the expression.."It's not over until the fat lady sings."