After evolution from menos to arrancer, the causality for power growth changed. Instead of eating other hollows for growth, the arrancer now grow based on experience gained in battle, just like Shinigami.
Shirosaki is the embodiment of Ichigo's instincts, which increase with Ichigo's desire and ability to fight, which is also gained via battle experience.
Ichigo's power grew significantly after battling Grimmjow, battling Nnoitra and watching Zaraki's love of battle and pure battle instinct. Ichigo died vs. Ulquiorra, then it seemed like his powers increased AGAIN when he came back to life as a fully transformed Vasto Lorde Vizard.
Watch episode 233 and 234 of Bleach. It's a filler, but it's about Shirosaki! Shirosaki's still an Adjuchas Lizard in those episodes, but he explains he's the form Ichigo's instincts takes up, and the source of Ichigo's hollow powers.