“…the scale of the object I'm morphing increases by something like 20-30%.”

 I don’t think what you are seeing is the “scale” of the object, but the morph being applied too strongly.
A dial setting of 1.0 is usally way too much for a morph done in Zbrush.
Try turning the dial to something less than 1.0 until it looks like it did in Zbrush…then set the max dial limit to that setting.

SWAMP