Interjection: The issue with these Halachos is that they are not explicitly stated anywhere. The closest we come is in Orach Chayim (somewhere in the early-to-mid 70s; 74 maybe) where the Shulchan Aruch says what is an Ervah that is Assur to say a Davar Shebikdusha when you see them. The reason is that since these areas cause certain thoughts more than others when men see them, therefore you may not say a Davar Shebikdusha when they are within sight. From that, it’s obvious to understand why they must be covered in public at all times. The point is they are much more likely to cause men to sin, therefore they must be covered. (R’ Schachter gives an example in several situations that revolve around the fact that the Gemara calls all of these body parts-thigh, upper arm, hair, etc.-an Ervah. He says that everyone obviously wouldn’t walk in public completely naked. Halachah considers showing these body parts being completely naked. So why would we treat them any differently?)
After that, though, the issue becomes a bit more muddy. Just from this, for example, wearing only pants or thick pantyhose would seemingly be okay. There would be no problem with having clothes that are way too tight. Everyone obviously agrees that these are problematic. So there is a second issue, aside from just being Halachically “naked” in public, that any clothes that are too attracting are not allowed. That there is no source for and is probably dictated by societal standards and common sense (i.e. even if society accepts something that is objectively too attracting, that wouldn’t make it okay; but if society says that something is too attracting, it would be problematic to wear even if it’s objectively not too attracting). And even though it would not be inherently Assur to say D’varim Shebikdusha in front of a woman wearing, for example, tight pants, the problem with that is obvious.