I don't what it's like to be an instructor and have your body "critiqued" directly or indirectly. But my take on it is this: for every instructor that is not a bean pole, you have a whole lot of people taking or watching the class who feel at home and take heart that you do not have to be uber-skinny to be fit, or to wear work-out gear in public. This is what people look like. This is normal variation. As is grey hair at 40 for that matter, or A-cup boobs on many women, all of which we forget if everybody tries to hide it or change it.

You're the best possible role model if you can inspire people, work on getting fit and getting them fit, and show your love of teaching without being the skinniest one there. You can even crack jokes about it, if that suits your personality.