Actually, the Argentine team on the weekend was heavier as a a pack than the Springboks, by about 30kg's.
also, Bismarck Du Plessis had a problem with his studs in the beginning and then replaced his boots with longer studs, specifically for scrum time.
Also the Argentine teams pushed prematurely a couple of times, which has been a tactic of them, with the ref penalising them often, which has been an oversight in the past by other refs.
There is not really much of a secret, they go low, and try to wheel the scrum as soon as possible to benefit them. with that said, their most recent tactics have been a bit more on the "cheat" side. but then again, most teams go that route all of a sudden.