Why do the laws of tzniut seem to focus so much on women and female clothing, and much less on men?
Perhaps this is because until recently, if a woman had an illicit desire for a man, it was unlikely to lead to anything, because women were not socially expected or able to start illicit relationships. If a man had an illicit desire, he was more socially capable of turning it into an illicit relationship. Thus, there had to be more protections against illicit male desire than against illicit female desire.
If so, then these days when women initiate relationships to a much greater extent, men would seem to be required to be more conscious and careful about their tzniut.
Nevertheless, female tzniut undeniably still has more objective limits than male tzniut. I doubt anyone will ever be checking male sleeve lengths, for example. Possible reasons for this discrepancy include:
1) The region of Torah-defined "nakedness" which simply must be covered is larger for women. (My pet theory is that for both sexes it consists of just the genitals - after all how much clothing was Eve wearing after the sin? - so on the "nakedness" level men and women have the same clothing requirements. As support, I bring the halacha of how much clothing women must wear to make brachot. But my pet theory could be wrong.)
2) When the rabbis at some point made the decrees which form much of our current laws of tzniut, they put some of the decrees for women, but not for men, in objective form. Why? I've given a few possible reasons in this post, and there are probably others.
3) Socially, there is a greater danger that women will try to "stretch the boundaries" of modest dress, based either on their psychology (which I don't understand), or on what men are perceived to want. Thus, objective limits are needed as a clumsy way of ensuring that women's clothing is more or less appropriate. (As an example: according to some, this is why women are expected to wear skirts. It is simply too hard to define what degree of tightness would make pants immodest, while any skirt of a certain length is more or less guaranteed to be modest.)
(Somewhat based on http://www.moreshet.co.il/web/shut/shut2.asp?id=29217)