I think you're thinking along the right lines. Muscle is denser than fat but the distribution is very different. Many sumou wrestlers have a lot of muscle but are covered in a whole lot of fat, too. I once saw a TV show in Japan that was comparing the average fat boy to a sumou wrestler of seemingly equal build. the sumou wrestler had a much larger percentage of muscle (I don't remember the actual percentages). Having a larger mass makes it harder for someone else to change your momentum, therefore making it more difficult for someone to move you some place you don't want to go. However, sumou wrestling, like judo, has a lot to do with throwing someone off balance by using their own momentum. In this way, someone with good reflexes who is quick on their feet will have a marked advantage in this respect.
As for your last comment, I saw a clip of a muscular maybe 200+ pound guy against a 400+ lb fat boy in a UFC event. The muscular guy kept trying stuff like grabbing the fat boy and punching him, but the fat boy just stood there and laughed because he didn't even feel the attacks through his fat. But eventually, the muscular guy literally climbed on the back of the fat boy and did repeated double-handed hammer blows to the base of the fat boy's skull until he gave up. The fat guy had a large amount of neck fat too, so it wasn't lethal or anything, but it took him out.