Knights and warriors - So we're dealing with a rather wide selection of roles here, even if we take an RP approach to the whole thing. And you ask what breasts have to do with it? Well, as you then go on to explain how distributing force across the human body works, you again mention the breasts being factor. And you were to one who brought them into the equation to begin with - But I'll say it again: Assuming that everyone had large breasts isn't really a thing. And I know how breast-binding works, thank you. Intense physical training and such will also decrease the amount of fatty tissue in your breasts, thereby making them smaller. That's why we have very few world-class female athletes with C's, D's, and anything beyond. If you want to maintain your body at post-peak physical conditions required for this kind of combat, you're most likely going to have more muscle than breast tissue at that point.
That aside - I'm also aware that wearing armor tailored for someone else is generally a bad idea, and rather uncomfortable - In fact that's why we have shoe-sizes I do believe. Because we're not all the same. I might not have the knowledge of psychology or experience in wearing iron plates for clothing to back that up, but I have reason to believe I'm right. The whole point of having anything tailored to you is to make it as easy for you to wear as possible, I do believe.
So, if we're still going to go with the classical, rather narrow idea of what a 'Knight' is, which is a rather futile exercise, then no - There would most likely not have been very many females in full body-plating, on horseback with a lance and a greatshield. You're right.
But the definition of a 'knight' is not that straightforward, and even if you go ahead and link to Wikipedia that's just not a good enough source. Knights are very different things in different cultures, even inside the same country. And that's without even mentioning the 'Warrior'-bit of the topic. A warrior is literally anything in any age.
Up until the iron age if you will, it wasn't a common thing for anyone to wear full body-armor of any type. Most soldiers serving in the different war-bands were given nothing other than what they inherited, this often being a tabard with the family or clan crest on it, a weapon - Sword or axe usually, and a helmet. Some had shields too of course. But that was about it. When looking on a timeline of war-history, armor as we know it and discuss it here is a fairly recent thing, and whom it fits isn't really all that relevant - But just to finish off this post: It's very possible for a woman to wear it and be as effective as a man.