In my experience (which isn't a lot), there's little relationship between size at birth and size in adult life. I've seen so many teeeney weeeeney kittens - one half the size of the rest of the litter - turn into bigger cats than their brothers and sisters. The biggest of several litters - almost twice the size of their siblings - grew into "normal" sized cats. One I've seen was SO much larger than his brothers and sisters turned into the largest adult kitty - but he also just looked a lot larger because he was the only long hair of the bunch.
Birth size doesn't determine much, especially since cats can get pregnant with multiple kittens at different times while they're in heat. So, when they give birth, the largest kitten could actually be many days older then the smallest kitten.
I raised 2 preemie kittens that weighed just 2 ounces and now they're normal(maybe a little bigger then normal) sized kitties.