A Larger grip might not be the answer (I came off cruisers and it took me a while to adjust to smaller grips).
The progrip model 714 is the best grip out there when it comes to comfort and alleviating pain or annoyance from overly vibratory bikes.
If you feel you still need a physically larger grip, but want something firmer than grip puppies then Kuryakyn makes very nice and comfortabel grips that are physically larger. They're the ISO grips. I have had them on half a dozen different bikes, they're great.