Smaller calipers on bigger rotors?

I'm gonna do a disc brake conversion with some stock parts on my obs. I was wondering though can i use a caliper from a 305mm rotor on a 345mm rotor or is the caliper actually bigger? Not sure if the actual pad contact area on a bigger rotor is bigger or just overall diameter.

almost anything works.. get the correct parts for the caliper and install them on the rotor and see how it sits... may have to adjust a little.. just dont want to to be to shallow. and only use half of the rotor...