Looks like a Ridley Scott thing.
If they could just make more spartans why didn't they do that after Reach?
they did with the Spartan IV program but it took a while to crank them out. Master Chief is a Spartan 2 which are the ones that good juiced up and experimented on. Spartan4's are like regular soilders in battle armor 2's are like walking tanks.
I think they're both juiced up, but Spartan 2's are indoctrinated from birth and 4's are volunteers.
They weren't as heavily modified on a genetic/physical level as the 2's though. 4's have some stuff done to their bodies but they're more reliant on the body armor and tech and weren't trained since childhood like the 2's were