Well at the risk of daring to use logic in Bleach. It was said that when bankai are first achieved. They are premature. Requiring 10 years of training atleast to master. Also that they typically grow with their shinigami. So initial release typically takes the form of being merged with the owner. Over time, more power can be drawn out as the shinigami grows in strength.
Hence why Hitsu's bankai seems to be continously growing.