My big guy is around 18 pounds and he fits in the dome just fine, though I think he did like it better without. Nevertheless, he still uses the Cat Genie no problem. This has cut down on the "litter flinging" so much! Even with the sidewalls, he was large enough to toss the pellets EVERYWHERE. With the dome, he has one way in and one way out, so the mat can catch what sticks to his paws. Don't order a Cat Genie without it if your kitty likes to toss litter!