User blog comment:KnocknickKnock/Lucy Stardress:Crux Form/@comment-25395791-20150520142624/@comment-24264679-20150520145842

The star dress works on all her Celestial spirit keys. It was never stated to only work on zodiac keys and the reason she hasn't used it on her silver keys is because there has never been a time in which it could have been shown. Also during the Leo arc Lucy asked Crux information about Leo and Karen in which Crux gave Lucy the story. Crux stated that he knows everything surrounding around the Celestial spirit world and those who have had relations with it. Zeref created the eclipse gate which needed the Zodiac keys meaning he at some point had a relation between the Celestial spirit world and him, meaning Crux would have information on that.

Crux delivered the information about the eclipse gate directly to Lucy's mind but in the case of her using the star dress it wouldn't matter as she would gain the knowledge as well without him downloading the information to her mind. Also Hikibi never downloading the magic to Lucy's head in the same sense as Mavis gave Cana Fairy glitter he just downloaded the knowledge to her brain which is why she said I can see magic circles in my head.

I not saying the spell would be powerful as the Celestial spirit kings but the size of the person has nothing to due with casting a spell just how powerful and big it might be.