Yeah, that's kinda impossible to have that because in school we learned that there are no medium genes, they get one or the other, the more dominant gene. if you grow bean plants and cross polinate a short one to a tall one, three offspring should be tall and 1 short. idk it's confusing, but i know the whole medium sized offspring thing is impossible.
Genetics isn't always as neat & prescribed as a chart in a biology text. Sure, there is the dominant/recessive thing going on with some traits, where parents displaying dominant traits may have a certain percentage of children showing recessive ones. But then there are other traits where the genes are more blended, sort of. It's been a while since I studied that in school, so I can't explain it more precisely. But it's the reason for redheads and hazel eye color in humans, and medium-sized chickens when bantams & standard-sized birds mate. And Ma Nature works it out so that a large chick doesn't develop inside a bantam-sized egg, & vice-versa.
I have a handsome mid-sized roo whose father was my Brown Leghorn & mother was a bantam.