The physics of running a wider tyre is such that lower psi will deliver a firmer ride and running too high a pressure on a narrower rim can cause extreme stress to the rium because of the wider section... they perform much better when you have them mounted to rims that are of the proper width.

I run Marathon Racers on my touring bike and they replaced the standard Marathons which were very firm at 65 psi... due to the racer being a little lighter and more supple it can handle a little higher pressure and still deliver a very good ride with excellent road handling.