Playing multiplayer is actually a major advantage numerically. You get a lot more total ships, a lot more total energy, a lot more total knowledge (much of which will be spent on duplicate techs, probably, but certain specialty unlocks need only be done by one player, etc), and even somewhat more metal because the game seeds more metal deposits in MP.
The AI gets roughly 2x the wave-strength on a 2-player game but a lot of other responses only go up by about 33% per extra player.
So numerically and mechanically it's very much in favor of multiplayer.
But the numbers don't make the most difference between MP and SP, player coordination does. Do you cooperate effectively? If so you can beat stuff that would be much harder or impossible in a comparable SP scenario. Do you cooperate poorly? You'll lose games that should be easy.