Physically, the contrasts are enormous. But then, Ivan contrasts with pretty much everyone on the physical level, being such a massive great hulk of a man. As a deaf mute, he's unable to speak, so inevitably lacks the eloquence of his master, the General. Zaroff is also quite a handsome fellow, in stark contrast to the gigantic man mountain Cossack with a long black beard down to his waist.
Zaroff may be a complete psychopath with a total disregard for human life, but he still retains the polished manners of a European aristocrat. Ivan doesn't really go in for manners much. But then he doesn't need to; he's just there to frighten, intimidate, and generally act as Zaroff's all-purpose thug. A giant he may be, but gentle he is not. Ivan takes after the General in getting a kick out of hurting people, except he's much less good at hiding it than his slick, sophisticated master.