In 8-ball, I choke up quite a bit for my break. I'm guessing that it could be because by choking up that at the impact point I have my hand much closer to my center of force. Possible that the shorter cue is forcing the breaker to mimic this? Try taking video samples of both and seeing what the difference may be.
For 9-ball, I find that a more smooth, controlled break works much better than the 8-ball slam. It may have something to do with the masses of the racks or the pattern of release from the rack, but trying to slam the 9 rack tends to produce a crappy (or random, at best) break for me.