you also forgot to mention the nervous system, and its speed of transmission and effeciency etc
pretty much genetically given, and can have a big effect on how strong and explosive someone is. And so their ability to get bigger and stronger is influenced from that to
Sprinters tend to be ectomorphs who stay naturally lean, who have fast and explosive CNS, with lots of fast twitch fibers and have lots of anabolic receptor sites where it counts. They don't look big but can blow up over night when they want to, especially with the nice receptor site advantage that can magnify the effects of drug use
so just saying wether someone is an ectomorph, meso or endo doesn't really make a whole lot of sense. Too simplified