On a side note I saw a thread that said the size of the snake is about which was originally used as the male and female and %SD doesn't matter as much. For example, take a pure SD male and mate it with a monster Mainland female. You will get lots of big eggs with big hatchlings that turn into big snakes. But if you did it the other way around, use a mature SD female and a young Mainland male. Now your eggs will come from the SD female and will be much smaller and you'll have smaller hatchlings and smaller snakes overall. I never tried this myself or have seen a direct comparison side by side but it makes sense to me.
So bottom line, you could have two 50% SD snakes coming from the same line of SD and Mainland and end up with two completely different size offspring (having different size potentials as adults) depending on which was originally the male and female.
Also, feeding has little to do with the final size of the snake. You can feed a corn snake as much as you want all day long and it will never get to the size of a boa, it's all about genetics at that point.