I'm pretty sure Hazel and Cha-Cha are enhanced in some way based on The Handler telling Five that they could get him a new body. As well, I've seen a few theories about his strength so I'll just list them here. Any one could be true or none of them could be true so take them with a grain of salt.
a.) That his super strength is directly related to his self confidence. Before the accident both as an adult and as a child he was the designated leader of the seven. This along with him being both smart and good looking would probably put his confidence quite high. Him both gaining his new body and going through heavy body dysphoria because of this new body would absolutely tank his self confidence.
b.) Him living on the Moon for as long as he did screwed with his muscle and bone structure. There's a reason Astronauts can only stay in space for limited amounts of time.
c.) It could be his new body doesn't have super strength. Just the (still very high) normal strength of a gorilla.