Why is there a weight difference between cats and dogs on the small PlexiDor? Cats are generally more comfortable going through small openings than dogs are. And, they’re built differently, so a 24 pound dog needs more space than a 24 pound cat. The small PlexiDor is recommended for cats up to 24 pounds and dogs up to 9 pounds. If your dog can comfortably pass through an opening that is 6 1/2 x 7 1/4 inches, the small PlexiDor will be fine. Cut out a hole that size in a piece of cardboard and test if your dog seems comfortable going through. If not, it’s better to go up one door size.