I am big guy myself. I can't do back flips or those awesome handsprings, although rolls and pinwheels aren't a problem. But I see it this way... It is about preventing injury, receiving an attack, or avoiding danger. IMO as your taijutsu gets better so should your ukemi. Essentially, with training, your body will naturally want to go to the safe spot, wherever that spot is or how that spot is shaped.
It seems as though I am on constant vigil to try and get smaller but I don't stop training. Don't worry about it and start training.
There is a difference between being acrobatic and being tactical or effective. With that being said, you should continue to push yourself to the limits of your capabilities in all aspects of your training.