He's likely a logical mage trying to figure out just what the heck happens to the extra mass when a human's shrunk for the first one. I mean, if I were a mage that's one of the main things I would want to know. Doesn't help if the mass goes to the center of the planet and it eventually makes the place so dense it collapses in on itself. "Faeries will be the end of Felarya," I say, but not intentionally!
About the producing offspring only Karbo the creator knows. It might be an impossibility, it might be possible but bearing sterile offspring, it might be possible with fertile offspring, only Karbo can give a final answer on it.