I am not exactly sure what makes a modern mainframe(same architecture as a historical mainframe I guess) but for historical machines I consider the wide machines(like the 36-bit pdp-10) mainframes, where minicomputers were usually narrower. 16 or 18 bit machines.

Really there is no good formal definition, dividing computers into three groups(mainframe, minicomputer, microcomputer) based on how much you payed for it, is as good a mechanism for figuring this out as any other.