Koko has a very different body type from Cain. She has had the matchstick figure since 2014 or even earlier, apparently without gaining or shedding weight but always remainings skinny but stable. She also supposedly ran low mileage until she was about 20, so her body had a longer time to adjust to strenuous training. (Accordingly, she was already 18 when she got really good with 4:09 and 8:53.) And she joined NOP a few months before turning 22, not with 17 or younger. Koko never had a stress fracture and overall hardly any injuries (except once in spring 2018). If anything, she gained a tiny bit of muscle in the year she has been with NOP as she is doing more weight and stability training there, she has not becoming skinnier with them. Whatever story it will be, it has been and will be different from Cain's.