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

[Kayleigh McMillan]

I am positive it is random I've heard multiple of the owners and CSRs stating that.
I think there is a sort of sequence ofcourse when the server hands out an odd size just as with specials vendors; the rare can come to you after one try.
It must be so though, due to the sequences, some are extremely lucky while orhers aren't: random at it's best.