Originally posted by: Prof. Chaos
Well remember when they first fought with Ulquiorra when Ichigo was fighting with his Hollow? How his spirit pressure was so great that he could have killed both of them with ease. Ulquiorra said so himself, however Ichigo was easily defeated cause of his battles with the Hollow inside.
That's because Ichigo's power rises signficantly when the hollow takes over. Remember that dude that was trying to get Ichigo to join them, he was a Shinigami that has let the hollow in him merge with himself and therefore get much stronger.