I'm going to go ahead and say that overall he does not change mass, he just changes density. In extreme cases like hannibal he would most likely make a smaller copy by comparison. That being said, his smaller copy would probabbly look about 400lbs. He wouldn't have anywhere near Hannibals strength but his claws teeth and scales as well as his flexibility would most likely be copied. His healing factor would be a nono and all his powers of adaptability would be much weaker versions if they even work at all. To a trained eye the difference would be easily spotted but to the average who is shocked at the fact that something like a giant lizardman is walking the streets would think they are one in the same.

I'm trying to make it that his power is more of trickery than stealing powers, but some physical traits would still be possible. Exe copying claws, teeth, scales etc.

Your lizard is freaking badass btw!!! I got my own lizardman in my character sheet cache that I playi some games myself if you want to peek in there :)