It looks like you have a 2002 model. If your bicycle was equipped with a smaller rotor on the rear wheel I would leave it that size. It is common for the front rotor to be larger due to the fact that it does more work because of weight transfer during braking. Thus more heat is generated, and a bigger rotor has more mass to absorb, and dissipate, that heat. It just isn't necessary, or helpful, to have a larger rotor on the rear. The brake pads have the same surface area front and rear. You won't be getting any better performance.