|So your washer doesn't do this with normal clothes? Personally, I don't think that 20-24 diapers is too big of a load, and it shouldn't phase a standard washer, because if you were to put in a comparable fabric area of towels, say, that really wouldn't be that many towels.
I guess I'd try using a smaller load setting, like the regular large, and see if that helps? I don't really see why the water level would affect how the diapers bunch up, but it's certainly worth trying. I would think that the large water level, or even lower (checking to make sure the diapers can move freely) would be fine, especially since you are doing that pre-rinse to get rid of the worst nasties.