These Petsoft diapers are the ABSOLUTE BEST diapers on the market! They fit my senior cat so well and do not leak through. She's 17 yrs old and her weight fluctuates from a mere 4.3 lbs - 5.5 lbs, and the XS fits her perfectly. The tail hole is smartly designed with a tight-fitting "X" which, with a pair of scissors, takes only 5 seconds to adjust a little larger to accommodate the thick part of the base of her tail. I don't know what I would do without these diapers, because the competitive brands just do not compare.