In fact jockeys pretty famously have to be UNDER a certain weight, and you will note that most jockeys are short and light. The weight is added to the saddle, not to the jockey. So as most women are naturally shorter than most men and weigh less, this would not be a disadvantage, particularly.
And in fact, for most of the time horse racing has been around, if girls were interested at a young age, they were dissuaded by the lack of adult women as jockeys (there have been a few; one of them later married Fred Astaire) and because it’s not seen as a really desirable occupation, although this happens for men, too.
This may be true to a point. I remember what Bruce Springsteen said about his sister: she was a far better musician than he was, but she had friends and a social life. She wasn’t going to sit in her room for seven hours a day playing guitar like he did.
That said, though, what about sports like gymnastics? Female gymnasts probably put in more time training than any other athletes (of their age, anyway). Gymnastics training regimens sound more like horror stories than anything else. I’d be tempted to say the same thing about classical violin - I’ve heard astonishing stories about how much time young girls spend practicing and how good they get. Are these exceptional because of some social convention?