User blog comment:Omega natsu2/Fairy Tail Episode 205: Beacon of Rebellion, Review./@comment-86.70.179.115-20141025100230/@comment-7418318-20141025100944

They did say "the power of an unchained celestial Spirit" which would suggest that they are somehow limited by being contracted and summoned by a Human.

And when Lucy was granted Aquarius' power, she did put up a shield that was able to block Jackal's explosions, even though Aquarius herself was only able to delay Jackal when he used the very same powers a minute before, so we already got two hints at Celestial Spirits having more power than Lucy is currently able to bring out of them.

It'd also be completely in tune with the whole concept of Spirits getting more powerfull as their summoner get's stronger.