User blog comment:Tekkphreke/Shikai and Bankai/@comment-4207271-20131224061221/@comment-5398287-20131224162402

Yoruichi said one's power would grow by 5 to 10 times upon achieving Bankai, but that's not what I mean.

Every, and I do mean EVERY Bankai we've seen thus far in the series has been far more powerful than its Shikai. Additionally, given that it's a higher stage in the Zanpakuto, it only makes sense that Bankai is inherently stronger than Shikai, much like how Shikai is inherently more powerful than the sealed state of a Zanpakuto.