I don't think there's any evidence to support any of those claims. Neither the claim that smaller maps and less civs are more suited for single player, nor the claim that they are less suited for multiplayer.
Multiplayer exists in a variety of forms. If anything the evidence points towards casual multiplayer having larger player counts and more variety, with competitive multiplayer having lower player count and lower variety (per game). But it says nothing of popularity.
As for single player, I don't know where you got that notion from? From a performance perspective alone, size demands more of your system, before you even throw internet connection, etc into it.