As with a lot of opportunity on buffalo, and probably scrub bulls (dunno about kudu), it often depends on whether you have been "made" or not during the stalk.
A big bull might look calm, but if he's spotted you he will have done an adrenalin-dump in preparation for fight-or-flight and a marginal calibre can get you into serious trouble.

If a bull is unaware of your presence at the time of the shot, a marginal calibre bullet in the right place can be just as devastating as a big-bore (as VC demonstrates!).