N64 ROMs vary from 4 MB to 64 MB (that's bytes).
Remember, that MPH client doesn't have any AI in it, and only one controllable character. Even smallish N64 games like Goldeneye had quite a bit deeper multiplayer modes, and those games also had fully featured single player modes.
Something like a Play-Yan for the VC might be entirely possible though... although since there aren't enough buttons, emulating the N64 would still be a dicey proposition.