In general the benefits of having more players seem easily to outweigh the drawbacks (more resources, more Homeworlds, easier Energy and Knowledge), this is probably an effort to encourage team play (it is more fun after all) 
This is mostly true, but the effort of coordinating all those players actually makes up for it in my experience. If you have four players who are all
really awesome and singularly coordinated, yes it's undoubtedly somewhat-to-significantly easier in multiplayer. The reality is that teams have uneven members and in general players don't all do things together and at once, and so a lot of inefficiency creeps in.
Think how long it takes you to decide where to eat lunch, versus you and three coworkers. That's basically what I'm talking about.
For most players, who aren't fitting that uber-skills/uber-coordinated pattern, it should be about the same in multiplayer and solo. If you fall into that first category, you'll want to turn up the difficulty slightly when you play multi. And if you fall into that category, I salute you: I and my alpha testers, who've been playing this game weekly since 7 months
before AI War even came out, don't fit that profile.
