If you REALLY want a full size grip on a J frame, look at the Pachmayr Decelerator grip. Full 3-finger grip with very cushy rubber over the backstrap. (They feel much bigger than the Compac model, and are made of softer, squishier material.) It'll make you wonder what that funny little gun is sticking out the top of the grip. But I think they feel great in the hand.
I haven't had K frame in the house for a decade, but I'd guess that the K frame version of this grip is the same size, just with a different grip recess inside. I can shoot a LOT through a J frame with these grips. But they are way too big and sticky to conceal well, if that's an issue.