After my comments earlier, I have reviewed my cattery. Though I started breeding when KittyCats started, I can't give an accurate statistics from my other avatar so I will give statistics form this avatar. I checked the total kitty boxes I have and I have up to 1575 as of June 7th. I have more, but I will not include it. I started breeding on this avatar April 2013 with one pair because I was still breeding on my other avatar. Eventually, I increased my pairs as my projects expanded.
I believe that the sizes are quite random based on the size of your cattery. To put my theory to a test, I put this together, based on what I have up to June 7th. For the first 4 rows, I did it by months and afterwards I calculated the number of kitty boxes and what sizes I got every week. The number of kitty fluctuated due to retired kitties or sent few to get kitty points.