The woman with armor is a popular trope in fantasy literature and illustration (although in fantasy illustration it is usually women without armor).
When this subject gets broached on the internet it is usually fraught with peril, opening up all sorts of accusations of sexism on the part of the author and/or various commentators. Therefore I will try to tread carefully. Historically there have been women who have donned armor and gone into battle. This is an undeniable fact of history. Boudicca and Joan d’Arc are testimony to that reality. However, it is also pretty safe to say that the vast majority of warriors throughout history have been male. Women warriors have been a minority if not an oddity in history.
This is not so much the case in modern fantasy. Although not always the majority, modern fantasy novels tend to have a preponderance of women in armor. The Warrior Maiden is not unknown in mythology. The Valkyrie of the epic sagas — most familiar to us today represented by Brunhilde from Wagner’s Ring Cycle — was a tradition that Tolkien borrowed from with the character of Eowyn in The Lord of the Rings.
Tolkien, however, was predated by Robert E. Howard who had a number of female characters who took up the sword and armor and were just as capable as a man. Despite the many who criticize Howard for being sexist, the pulp writer from Cross Plains was, in fact, ahead of his time in his attitudes towards women. A writer who created characters such as Valeria, Dark Agnes and Red Sonya of Rogatino cannot, in all fairness, be characterized as sexist.
From the pulps C.L. Moore created Jirel of Joiry, and Lin Carter had Tara of the Twilight. At the time they were considered little more than a literary novelty act but in latter times have been adopted by certain factions as proto-feminist heroes.
Other women warriors in modern fantasy range from the realistic to the outright impossible. From Howard’s Sonya of Rogatino comic book writer Roy Thomas morphed her into Red Sonja. This Sonja was born from the outrage of rape (echoing the historical Boudicca) although she moves quickly into outrageous territory with her predilection for wearing very scanty chain mail.
It is certainly absurd to go into battle wearing little more than an armored bikini, but artists tend not to dwell on realism, preferring to dwell on womanly curves.
Touted as being more realistic is George R. R. Martin’s Breanne of Tarth, and yet given the historical reality of women warriors Breanne is unusual. She is physically unattractive and she is huge and super strong. She is also the ONLY woman to don armor in all of Westeros. She is an oddity and the recipient of much scorn and abuse. The conventional wisdom is that she is a more “realistic” portrayal of a warrior woman then, say, Xena, Warrior Princess, yet given the examples from history — Joan d’Arc, Boudicca, Rani Lakshmibai of Jhansi – Breanne is less realistic than Xena.
I’m not sure if I’m getting anywhere near a point in this ramble of words, and I certainly don’t mean to be reductionist, but it is a topic that is fascinating to me (and I’m not ashamed to admit that the unrealistic fantasy images are some of my favorites) and further exploration, both in terms of research and writing is warranted.