User blog comment:Xilinoc/Zanpakutō & Tōjū arcs - Review/@comment-493809-20140122162609

I had expected Muramasa to become a Hollow at some point, since my understanding of Zanpakuto Spirits is that they're the inner power of the Shinigami focused in a safe form, while the Hollow Form is that same power manifesting around the soul, becoming the dominant one and taking over the soul. Similarly, the fact that he needed to devour Hollows to survive seemed like a direct parallel to Menos devouring one another. The power difference likely came from the fact that while, as a Zanpakuto, he was used to limiting/conserving his power, when he turned into a Hollow he became largely Id-driven and lost the ability to control the souls - that, or he retained his ability to make things manifest, thus allowing him to summon the consumed Menos.

Not defending Koga for a second, though.