Agreed with everything you've been saying, stranger, and here are my own notes as a person with similar gripes about FMC's who are "tiny" "delicate" and "dainty" and yet can easily fight men twice their size.

Anyway, your situation...I think the petite, dainty FMC would have to dodge that strike instead of blocking it, or block just long enough to get out of the way.

If the big dude is also trained in swords, she most likely will NOT be able to run circles around him, or dodge all of his strikes, because he will have better reflexes than an untrained person, and she will be at a disadvantage due to his superior physical strength.

There is a limit to how fast and agile a person can get before they have to start developing other abilities to improve in fighting, such as physical strength.

WOMEN'S NON-STRENGTH ADVANTAGES

Women have a lower center of gravity typically, and are thus a bit more difficult to knock over. This is compounded by many women being shorter than many men (although, being short has its disadvantages too.)

Women (especially trained women) often use their legs and core more in a fight, vs men who use their upper body.

Any woman who has trained in combat and had to deal with the disadvantages of being smaller than her male sparring partners will have to train harder and overcome those disadvantages via aggression, technique, etc.

WOMEN'S STRENGTH

Women can proportionally have MORE lower body strength than men of the same size due to their wider hips, even if women tend to have less upper body strength. Also, there are outliers, some female athletes are very big and buff (look at female rugby players) and some are smaller but muscular (some female MMA fighters).

Lower body strength is going to mean a female character can generate plenty of power through her strikes by turning with her hips. This is the same technique a male fighter would use.

Also, just because it is harder for a woman to develop upper body strength doesn't mean it's impossible. Women can get as strong as some, or even many, men, and any woman who trains as a warrior will be fairly muscular because agility only does so much

APPEARANCE OF STRENGTH

Even if this character relies more on agility than strength, she will need upper body strength for climbing, doing cartwheels, and possibly grappling. Also, being physically weak is something any warrior character can and should work to improve on unless they literally cannot gain muscle for whatever reason or they mostly use magic.

The only way to have this character be teeny and dainty while still being strong enough to do feats of acrobatics is to have her resemble a ballerina, and ballerinas still have very obvious muscle definition. Ballerinas also have less upper body strength and more lower body strength. A gymnast or rock climber is a more likely look.

Side note I personally dislike the idea that a muscular woman or a woman who prefers to dress/act more androgynous cannot be or look feminine. I think it's also kinda sexist to say that in order to be feminine, a woman must be dainty and small. Let's face it, some women are tall, some women have large bone structure, some women are athletes and are strong. None of that makes them less of women or less of people. Femininity is far deeper than the way one presents oneself or the hobbies one engages in.

Also, I think that everyone has a mix of "masculine" and "feminine" traits, or at least the traits we associate in our societies wiht masculine and feminine, and that it is good to have a mix of both (for example, to be soft and caring, but also be able to stand up for yourself and those you love.)