User blog comment:Tekkphreke/Shikai and Bankai/@comment-4207271-20131224061221/@comment-11051098-20131225012149

Since Shikai is the 1st form of the Zanpakutô and Bankai is the 2nd form, it only makes sense that Bankai is more powerful. Considering what Yoruichi said, a Shinigami's power grows 5 to 10 times in Bankai release vs. Shikai release. The way I read it, Bankai being the 2nd/final form means that it has a far greater potential/range of power than the Shikai has.

It would also depend on the Zanpakutô itself how that works. If you look at Hyôrinmaru, Hitsugaya stated sometimes that his Zanpakutô probably is the one with the least difference between Shikai and Bankai because the only difference is the amount of ice it can create.

I think the same goes for Senbonzakura. Its Shikai and Bankai looks the same, they're both made up of a huge number of tiny blades. The only difference is that the Shikai consists of 1000 tiny blades, while the Bankai consists of 50 million tiny blades.

Zangetsu seems similar, in that the power of the Shikai is suppressed into a smaller blade in Bankai form, which increases its power and speed.