Board Thread:Versus Debates/@comment-28203534-20160509153448/@comment-734582-20160517105534

Rachin123 wrote: And I don't recall that the number of tails indicated their power. It was hinted but never stated that the number of their tails indicated their power. Kurama believed tails indicated strength, and the fight against Obito suggests it's true.

That being said, there's a correlation between abilities and powers here: Gyuki and Kurama both have the greatest raw power but have no real abilities of their own (ink and sensing, really). Compare that to Shukaku who has the sealing marks, sand for his body, Magnet release (and so two elemental natures). All Biju below seven tails have some form of element.

So the Biju seem to have a sliding scale between raw power and ability.