Are fat people the same species as normal people?

I'm confused... What is the difference between species and genes? Are fat people separating from the human race? What happens when a fat person has babies with a normal person? Will the offspring be sterile like the offspring of a horse and donkey, i.e. a mule? Will the fat/skinny baby have any special abilities like a mule has? Mules are unable to reproduce but have greater strength and better temperament than horses or donkeys. Will fat people or fat/skinny crossbreed people be able to vote? Do you think they should be allowed to vote?