In career mode you have a cost constraint to optimize for. I have noticed that certain rocket sizes are much cheaper then other ones. Using the Kerbal Optimal Rocket Size calculator,assuming a 10t payload and assuming it's correct, you would get values along these lines:
Total dV
Total Cost
Cost Next dV
9000
248630
/
8000
161570
87060
7000
88400
73170
6000
53160
35240
5000
30800
22360
4500
24470
6330
4000
21080
3390
3000
11200
9880
2000
7575
3625
As you can see from this, not very scientific, table the cost from going from 7000 dV to 8000 dV is much greater then going from, say, 5000 dV to 6000 dV. There is that strange anomaly around 3000 dV. There appear to be a brake-point somewhere when it becomes more cost effective to launch multiple rockets, for example a main one and a refuel one, then to launch just one single rocket. This also appears to get much worse the heavier the payload.
That is why I have been thinking about how a mining base on Minmus is a real gateway to the heavens. Allowing you to launch much smaller rockets because you can split the dV need for the mission and the initial to orbit and out of Kerbin's SOI.