So Akira gets stronger the less goblins there are, and he gets weaker the more of them are. I don't know if the human royalty knows about the way goblin life-and-death works with regards to passing down skills, but this is actually correct. Because when a goblin dies, Akira/the goblin king absorbs their skill back to himself and it evolves depending on the number and level. When a new goblin is born, they instead inherit the skill of the previously deceased so it doesn't evolve unless that new goblin is able to make it evolve by themself.