The best explination I can think of this is that the Yondaime used a slightly different variation of the sealing jutsu (I'm pretty sure that the Sandaime mentions this during the fight with Orochimaru), so considering this, and the Yondaime's enormous power he probably could've had the strength to seal the Kyuubi into baby Naruto.
Also, for those with more questions, www.leafninja.com has tons of information (their Connections Guide alone is a sight to behold), and it just might answer your questions.