I know what you mean. Look at Harry Aikines-Aryeetey, type his name in google, he is apparently 82kg/ 181lbs but could be confused for a mini Ronnie Coleman! (BBer)
I think it’s a low bone density as well. As far as I know, it doesn’t mean your calcium intake isn’t enough but is genetic. Thats what another sprinter from a different forum told me. 
Hope this kinda helps. 