Not necessarily directly related to your question, but a conversation I had with a friend who was an aerospace engineer, and who worked on solid rocket design:
Remember the Ares I/V designs? They theoretically were claimed to use what were "essentially" shuttle solid rocket boosters (SRBs), but scaled up. In fact, much of the chatter in general forums was that the Ares boosters were shuttle SRBs with an extra segment on top. (SRBs are built of several independent segments of propellant with the external structure wrapped around them. When they're assembled, they segments are stacked with, among other things, o-rings between them---that is what failed in Challenger.)
Ok, so you can just stack another segment on top for more power, right? Well, no. SRBs don't burn from the bottom to the top; there is a hole from the bottom of the propellant to the top, and when ignited it is the propellant on the surface of this hole that burns.
This hole up the propellant (remember, it's a solid) is not just a round cylinder. If it were, when you lit it on fire, it would start out at a minimum thrust and the thrust would increase as the hole got larger in diameter. This is not so much what you want a rocket engine to do. So the hole is actually a weird star shape that maintains the same surface area as it burns, which requires a bunch of strange parameters known only to the initiates of aerospace engineering magic. (This is what my friend did in grad school. Now he's an enterprise webby project manager thingy.)
If you just stack another segment on a SRB, it will burn for the same amount of time with more thrust, which may not be a good idea---remember, this is a mechanical structure that is mechanically connected to whatever you want to lift and the whole thing may not like higher g-loads.
So, to just "add another segment" you have to redesign the propellant entirely, then look at the structures and connectors that may not be designed for the new loads, and so on.
Scaling up isn't easy.
If I remember correctly (See the book Ignition!), one of the major problems for the Saturn V's F-1 engine was the design of the engine injector plate, the thing that sprays fuel and oxygen into the combustion chamber. It had dramatic effects on the stability of the burn (they can and will go BOOM!), the vibration and other aspects of the engine. And that was basically determined experimentally. Also, keep in mind that the big design problem for a liquid-fuel engine is how to feed as much liquid propellant and oxidizers into the engine at the rate necessary for the thrust. They use burning rocket propellant to drive turbine pumps to feed propellant into the combustion chamber, and those pumps are the cutting edge of high-temperature, high-pressure pump design. "Just scaling it up" can be a completely new research and development project.