Obviously, an infant of their species weighs as much as a full-grown adult in ours...in this case, about 160 pounds. (I'm still not violating conservation of mass/energy.) This first transformation was involuntary...but that'll change.

The Zha'Tahn colonized Jupiter, Saturn, and Neptune, but they're really extrastellar---from beyond our star system. They're visitors, colonists, what-have-you.