Every 7 kittens born, 6 are normal size and 1 special size!

[Jiyulie Resident]

I think different avatar has different data. After looking at my pedigree, I realized that my special sizes tend to come from special sizes parents.
Like from my teacup couple, I've gotten bigger de big, mega, toy and teacup and a petite. And it only has 7 offsprings so far.
Same goes to my 2 other pairs of toy couple and teacup-toy couple.

Therefore I conclude that for some, the sizes are "bred", some has a pattern and others are just random. As long as my couples are of special sizes together, I get more special sizes than normal.

So I think size variation is dependent on the avatar, the time they join kittycats and what system they get for breeding.