User blog comment:Omega natsu2/Fairy Tail Episode 213: Erza vs. Sagittarius: Horseback Showdown!, Review./@comment-7418318-20141220111325/@comment-7418318-20141222095454

Considering how Virgo is able to change her appearance from the get-go and how Loke sometimes has cat ears and sometimes doesn't, I wouldn't be surprised if all Spirits (or at least Zodiacs) can change their appearances.

Now the question is whether Lucy is somehow determining their appearance subconciousy (with her surprise about Virgo's slim appearance it's clear she doesn't do it consciously) or if it was the Spirits' choice to appear the way they do.