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. A few examples of similarity:

Hitsugaya have stated that his Zanpakutô is probably the one with the least difference between Shikai and Bankai, because the only difference is the amount of ice it can create. Its Shikai and Bankai are 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. All the power of the Shikai is suppressed into a smaller blade when Bankai is released, which increases its power and speed drastically. Its Shikai displays body parts of a giant. Its Bankai displays the entire giant.
 * Hyôrinmaru
 * Senbonzakura
 * Zangetsu
 * Tenken

An example of the opposite could be Suzumebachi. Its Shikai is small and kills in 2 hits at the exact same sp0ot. It's Bankai is completely different, far larger and heavier and having a far greater amount of power. It's also different in that it kills in only 1 hit.