User blog comment:Tekkphreke/Shikai and Bankai/@comment-5398287-20131224011305

I don't think it's a matter of power, but more of learning the true extent of the Zanpakuto's capabilities and learning how to control it. Essentially, the Shikai and Bankai of a Zanpakuto have different levels of power and (usually) different techniques: a Shinigami, through training and whatnot, can hone the release stages to their fullest potential. It's not so much an increase in power as it is an increase in capability.

As for the Shikai being stronger than Bankai bit, no. They may be more useful than the other in different situations, but there is a massive power difference between Shikai and Bankai. Even in the case of Hitsugaya's Hyorinmaru, whose Shikai and Bankai are very close in power, its Bankai produces a hell of a lot more ice.