Q. My older overweight cat has less litter box use than younger cat
DR. CHRISTIE LONG, D.V.M., C.V.A.
A. If you're wondering about whether your older cat has a medical problem because its output is less, then that's certainly a valid concern. Cats are by nature not very "robust" drinkers of water, and sometimes we have to encourage intake with a kitty water fountain, or by adding canned food to their diet. If you're wondering about defecation issues, cats can definitely get constipated, and if infrequent defecation habits are allowed to progress it can become a chronic problem leading to an issue called megacolon. You can try adding some canned pumpkin to your cat's diet (it's OK for both of them to have it) - about a tablespoon over food over meal is sufficient to add some fiber and hopefully encourage more normal defecation.
A. It may be more difficult for the heavy cat to get into or out of the box, making her reluctant to use it. It may be too small or not clean enough to suit her, cats can be very particular about their litterbox. The first thing to do is to get her checked by your vet to make sure there isn't anything medical causing her inappropriate elimination. That would be an excellent time to talk to your vet about a weight loss program. If she checks out clean, try getting another litterbox with lower sides and open top. Place it somewhere out of the way that doesn't get too much traffic and keep it scooped at least daily.