Mounting the shock on the inside reduces the effective spring rate and shock stiffness, because it increases the angle from vertical.
If the spring was moving in the same line as the wheel, for every inch the wheel moves, the spring moves an inch also. If it was at a 45° angle, the spring might compress half as much for the same wheel travel. So if you have a 200#/in spring, it would take 200 pounds to compress the spring one inch in the first scenario, but only 100 in the second. If the spring was completely horizontal, it would essentially show a 0#/in rate at the wheel, and just fall over on that wheel (I like to think in extremes).
I don't have a lot of racing experience, so there could be more to it that I don't understand yet, but I did get an A in statics