So far the only help I've found to do with this is for when a displacement map tears objects along its seems, which is to keep the alpha down as low as possible, do close bevels on the edges, and soften normals. As for not expanding the whole thing I dont know. If the map coming from z-brush is a 50%grey type background then (and im just thinking out loud, cause I don't really use z-brush that much), but can you take the map from z-brush and put it in photoshop and darken up the bits that aren't bumpy, or you know, just generally darken the whole thing, that would decrease the inflating, if not get rid of it?