It's all about the frame. Some glasses are really cute on a girl (normally round frames).
Almost stole the words from me.
Small frames are nice too, plus if you look at the librarian fetish for answers, glasses are an article of clothing to those people. Taking the glasses off (usually while also letting down their hair) is the same as removing their shirt and bra when it comes to them, and I kinda get why even if it's not overtly my thing.
Glasses cage the face, which can make taking them off to appear more nakedly fem all the more effective, but yeah small frames can be sexy too.