Hello XXXXXXXXX.
If the cat is 'very large' I am wondering if the kitty is overweight and possibly diabetic, has a urinary tract infection or intestinal parasites? If there is a litter box issue, please have the cat examined with full bloodwork and fecal analysis. If those tests are normal, then you may need to change the kitty's diet, gradually, to find one that better 'agrees' with the kitty. You might also consider adding moist food to the diet, which can help dilute the urine, thus help control urine odor. I find that certain cat litters will also help more quickly absorb and cover odors, especially if the kitty is willing to bury their eliminations.
Good Luck!
Dr. Melinda
Cat Veterinarian
10 years experience with small animals, special interest in cats and acupuncture