It depends where you go.  At nicer studios where most people pay per month and drop in classes are expensive its rare to find really obese persons b/c they are just not that motivated.  Most people have some decent fitness and if they need it they will take breaks when it gets intense and then jump back into practice.  The whole class is motivating and fun and you don't want to be that person on the mat.    What happens often is you get regular fats and small fats and lots of young 20 somethings and beer belly dudes from the frat house looking to cure their hangover.  They can't cut it at all.  They are down after like five minutes.  And they get up for a few minutes.  And then they go back down.  And most don't return.