Doesnt matter if you own admin spawned in gigas. They get 7% melee/level. An usually imprinted giga is just getting 3% melee/level. Just send a ticket to wildcard tell them that you lost a giga.
The most recent health nerf I remembers was from 25k base tamed health to 17k base tamed health, there was another one before that but I don't remember what the numbers were
The first one reduced its base tame hp to 25k or around there from 80k then it was reduced to 17k damage and hp have been merged a lot but those the biggest I saw