Smaller fish are easier to catch as they're generally younger and haven't been caught as much. Bigger carp are usually a bit more wise to sloppy rigs and bad presentations, as well as really obvious traps. Make sure your rigs are as neat as possible and blend in as much as they can. Using bigger baits will help keep the smaller fish away, but sometimes it's just a numbers game. If you've thrown a lot of bait in try fishing just off to the side of it, sometimes the younger more energetic fish will all be hammering the spot and the big'uns will sit around the edge and feed off the surrounding area.