Actually a formfitting costume can be justified as providing support and freedom of movement. That's why athletes wear formfitting garments. And particularly a futuristic, high-tech garment would benefit from being formfitting in that it could provide dynamic support to the muscles and joints, wick away perspiration and dirt, regulate body temperature, and the like. The only benefit of a looser garment, in that case, would be modesty, which is an arbitrary cultural convention that someone like Seven of Nine or T'Pol, would have little regard for. It is, after all, illogical to compromise efficiency and practicality for the sake of the emotion of embarrassment.
Although, admittedly, that would be far more plausible an explanation if we had seen male Vulcan officers dress in equally formfitting garments. For me, the problem isn't that Seven and T'Pol dressed that way, it's that nobody else did.