Antidote X question

Why, when Mojo was subjected to Antidote X, did he not turn back into a normal chimp? Why did he return to the large-brained, green-skinned, but otherwise small and weak version of himself after the first time he was exposed to Chemical X?

Originally, I wondered if it might be due to the length of time between the two X doses, but this was disproved when the Powerpuff Girls almost erased their powers using the remaining antidote.