User blog comment:FairyRaven/Fairy Tail Chapter 356: Tartarus arc - Prologue, Review and Commentary/@comment-180.254.175.89-20131018140531/@comment-7418318-20131018150107

Tartarus was veiled in too much mystery to properly judge their strength.

And even if Grimoire Heart really used to be stronger than Tartarus, that was seven years ago. Plenty of time for the balance of power to shift.