It appeared that an atmospheric change caused them to change physically almost instantly. Look at how Kal-el became weak on the Kryptonian ship and passed out and coughed up blood almost instantly. They got powerful almost immediately on earth also. Do they get full power immediately or just mostly? Don't know.
Concerning Kal's ability to match Zod: I took it as Jor-el was pretty tough, and Kal was a little bigger and probably just a genetically above average guy to begin with and that helped him compensate perhaps. And as already mentioned, maybe he got a lucky upper hand when Zod went nuts.