No, the other way I think. I don't actually know, to be honest.
You automatically get free units based on the percentage of you population (I think it is 24% or somthing, but I am not sure). Vassalage gives you an additional 10% or something. The problem is, because the civic cost is also based on empire size, and because the civic cost is high, it essentially makes it go one for one in expenses (I think the maintanence is more expensive than the amount of free units you get, but again I am not sure).
The problem with a large empire, is that you need a bigger military to protect it. So I don't know what's better: A bigger empire or smaller empire. One thing that might be of interest, is that the Vassalage civic may have an interesting impact to costs when expanding through conquest. When expanding through conquest, you get essentially free population points that will increase your free unit count. But that will also increase your civic costs too. So I don't know.
Bigger cities would be beneficial as the city could look after it's civic cost easily. I think the population growth out grows the rate of civic maintanence increase so it could be better to have big cities. That way your free units may out do the civic costs required to have the civic.
That would be my answer: Bigger cities equals a better ratio of free units to civic costs. I wish someone out there had more insight into it though.
Watiggi