I do kickboxing / Mixed martial arts and I love it. As an instructor I wouldn't be concerned about your weight, since if you felt like you really couldn't do a certain piece of the class, then don't do it. All I ask is for my students to try to the best of their ability. If I can see your working hard then that's fine by me!
You might want to go and watch a class first, but just get in there and try it - even if you do 10 minutes of a class, it's better than none at all