Just to note: Killing half of the kittens will NOT gradually decrease litter size. The cat HAVING the babies will have no survival benefit for having less babies, only the babies will recieve said benefit. And since the babies have no genetic influence on the number of kittens in their own litter, no evolutionary reaction will take place.

Now, if you killed all cats AND all their offspring, or had them fixed, if they had a litter above a certain size, THEN the litter size would decrease. But that's a tad bit on the impractical and unenforceable side.