Double standard is part of it, but sexualization isn't just about how much skin is showing. This outfit shows about as much skin as the Conan cosplay, but is clearly more sexualized:

A Supergirl or Powergirl cosplay can also be revealing, and while you might find someone in that cosplay (or the Conan cosplay) sexual attractive, they're not as sexually suggestive as what this girl is wearing (which is essentially some belts held together with body tape, I presume). The line is blurry and dependent on context, but I would say this is over that line.