Both muto was much more powerful than anything on the island.
It also doesn't matter if he's still growing since we don't know what he's gonna grow into.
Point is the smaller one still had enough power to hurt Zilla. You continue to make excuses. When Kong throws trees at him it's going to hurt.
So you don't believe he will be a peer in size. You need to have your hand held because you aren't intelligent enough to predict the obvious.