The source for tight clothing is anything but clear. I once saw it brought down from a Rashi in Masechta Shabbos about tying knots. They would lace up their garments or dresses instead of zippers. The gemara was discussing tying on Shabbos I think. Sometimes (maybe because of problem of tying on Shabbos) they would leave the garment tied. The gemara says a woman would not tie it too tight because then they would have to wriggle in and out of it, and it would take extra time to dress or undress. That would not be tniusdig. The gemara did not mention anything about while the garment is on the person it looks untzniusdig if it is tight. The problem was simply getting it on and off.
If anybody remembers the daf, please post. Also, if anybody has any other primary source for tightness being a problem, please post, as well.
Not everything that is attractive is untzniusdig. I mentioned orthodonture, as an example. Is getting beautiful teeth also a problem?