I know a lot of people argue when I compare Kilrathi to Kzinti (since the Man-Kzin Wars books came out first and to a degree one is based on the other)
http://emedicine.medscape.com/article/1117825-overview said:Supernumerary nipples (SNs) are a common minor congenital malformation that consists of nipples and/or related tissue in addition to the 2 nipples normally appearing on the chest. Supernumerary nipples are located along the embryonic milk line. Ectopic supernumerary nipples are found beyond the embryonic milk line. The embryonic milk line is the line of potentially appearing breast tissue as observed in many mammals. In humans, the embryonic milk line extends bilaterally from a point slightly beyond the axillae on the arms, down the chest and the abdomen toward the groin, and is generally thought to end at the proximal inner sides of the thighs, although supernumerary nipple has been described on the foot.1 Supernumerary nipples can appear complete with breast tissue and ducts and are then referred to as polymastia, or they can appear partially with either of the tissues involved.
So, uh... Yeah. Well anyway, I guess it doesn't really matter whether it's two larger or all large or all small, since we have evidence of all of these scenarios in real life. If you look at cats and pigs who have a litter, then all their normally small nipples are enlarged for feeding whereas otherwise they're just little bumps. You never know... maybe the priestess has a litter she's feeding at the time of that picture!
Ultimately though it comes down to whatever the artist felt like since I don't think there's anything to say differently. And... Kilrathi are aliens and not mammals, so there's no reason for them to follow the pattern of Earth animals at all.
I know they were trying to be true to a cat-female (with six or eight breasts) but really... six C-cup breasts? Her poor back!