I did my research on this one, and I'm pretty sure I know the right answer. However, I didn't find anything that specifically addressed this situation, so I'll spell it out and get the official take on it.
Shrinker. Extra Leaping. Villain on a rooftop two stories up. Shrinker elects to do a Move-Through on the villain, and wants to add Growth Momentum to the damage. I knew this wasn't growing up under the guy's chin, but I was in the middle of a heavy combat, and didn't want to stop things to look this up then. I ruled on the spot that the shrinker could do STR/5 + velocity/3 + 1/2 growth momentum damage, with the reasoning that the shrinker had nothing 'to push against', like the ground. After looking in FREd, the FAQs, and searching this forum, I believe that the growth momentum should have been disallowed altogether for someone in mid-air, regardless of orientation of attacker and attackee, or even to be added to a Move-Through.
Your take?