First of all, a city can only work 21 tiles, including the city center. So once your city is size 20, the extra citizens become specialists, and won't really help your city at all. So it's better to have size 20 with mined grassland than size 30 with irrigated grassland. The terrain also has a big effect. You can't expect to get Modern Armors every 1 - 2 turns. Since they cost 120 shields, you should be able to get them in at least 3 turns, or 40 spt in your cities. (That's 20 shields per turn in a railroaded city pre-factory and coal/hydro plant, not too hard to get).
ICS is CxC, not CxxC. What some people do is do ICS in the early age, get lots of land, and when it goes farther in the game, disband the middle city; for example: CxCxCxCxC becomes CxxxCxxxC, allowing for greater tile useage and more production. The main point of ICS though, is for domination or conquest, an early win. Most people don't do an ICS space race.