A lot of rotties have weight problems, but that is the owners fault. I have seen way too many that were very over weight. Rotties are not supposed to be fat. They are supposed to be pure muscle. I completely understand where you are coming from though.
I have seen many rotties that could run just as fast as any dobe. I think their mass fools some people. People think they are slow, but they could easily catch a running person any day! Bears are huge and they have a lot of body fat, but they can run up to 40 mph.
I think a rotties head looks like a cute stuffed bear when the are puppies, but not when they are full grown. Then they look like a grizzly bear, lol. If I had to choose... I would rather be attacked by a dobe than a rottie, just because they are sooo powerful, and their jaws are stronger in my opinion.