User blog comment:WhiteArmor/The Masked/@comment-452405-20100415085659

I wonder if it's their level of control though, Ichigo's mask just completely changed and during that time he had no apparent control at all. It could be the amount of time the mask is used though Ichigo's has grown so much in only a few months while Shinji's is almost the same 110 years on. Or it may be the reliance or amount of power drawn from the mask, Shinji and the others have just been training, not really a life/death situation to require pulling out more power from the mask. While Ichigo on the other hand has been fighting for his life almost the entire time since he gained access to it.

Maybe its how close the hollow's influence on the soul is getting or syncronising with the user's own power that allows it to grow. I know it was anime-only line but in the Zanpaktou Arc Ichigo claims that his inner hollow may of been a hollow to begin with but has since become a part of him.

There's quite a few ways this could go isn't there haha.