It's one of those standards - when a costume is designed for a man, it emphasizes beefy. When the same concept is repurposed for a woman, it's made sexy.
Even when it's a character like Spiderman, who's lithe in the comic books - the boy's get beefed up:
And the girls get sexed up:
But that's not fair, right? I used a kid's picture for the boy and an adult picture for the girl. Well then, how about this?
Boy = tough, girl = sexy.
Even when it's something like armour.
Look at Boba Fett's armour, as it appeared in the movie:
And a female version for Halloween:
Everything hugs. The armour plates - which would be functional in the original - changed. Spandex is added. And that's a tame one:
You can say this is what the market wants, or it's what men want, or it's all that's available. The point is, the trend exists - men's outfits repurposed for women get sexed up to some degree or another.
Which begs the question - what will Captain Phasma's Haloween outfits look like as the character, a woman in armour, gains traction post-Force Awakens release?