I would say that the dream game is what will keep you up at night, working to get it done. This kind of motivation is a powerful thing.
However, you will also learn a lot from making lots of small games. Do you really want your big magnum opus to be the first thing you make? How bad will it be, if you have not learned from your previous mistakes? If it was the first time you packaged/marketed it?
I’ve coded a noughts-and-crosses (tic-tac-toe, for you colonials 
) game, and learned plenty from that. About code re-use, refactoring, how to document it, so you remember what you were doing when you put that semi-colon in, at 2am on Monday morning last month.
My favoured Hitman game is currently my side project, as I learn about the systems that I need to make it. Those systems are making other smaller games.
I guess what I am trying to say is don’t look at these small games as diversions. They are stepping stones, getting you to where you want to be, just not as directly as you would want.
Good luck!
P.S. If your girlfriend is not your best friend, you’re doing it wrong. (I know, it was only an analogy )