I am agree with everything said on here, bc in my opinion its all true. The way you look does not mean you cannot deliver a good class with passion.
BUT, lets not forget that this IS a fitness class. and having that stereotypical body IS generally a bi product of being really fit. Can someone who is 20lbs overweight deliver a great class? yes. Could they deliver an even better class if they didnt have that weight? Yes. you may think that you give max intensity, but be honest, do you have the same intensity AND personality at the end of the class as you did in the beginning. I teach with intensity foremost, but when I get winded the personality goes out the window(as i cant breath). I admit the potential for me to deliver an exceptional class is higher when I am in greater shape. The potential.
i just wonder if some instructors hit a plateau in thier fitness levels, and instead of trying to break down that wall, they fall back on the fact that people are all different sizes. again I agree with this, but never use it has a scapegoat or a reason to become complacent. Remember "get fit, stay fit, GET FITTER!!!!!" That last part is for us instructors.
keep pushing!