So you are sure your master cylinder is working properly? You have fresh clean fluid and no air bubbles in the lines? The cable from the yoke to the master cylinder is adjusted optimally? If those other items have problems, a different caliper will not solve your problem. If you use a caliper with more piston surface area, it increases leverage on the pads, but the lever will bottom out before enough fluid is passed. If you use a caliper with a smaller piston area, it will get harder to pull the lever.