I don't think that's the case at all necessarily... she probably just got it cut before this and in the shoots where she had 'voluminous' hair, it was probably a weave/wig.
Quote:
As her presence as an artiste grew, she also started to gain influence as a fashion leader, and started various trends while in her early 20′s. Even that holds some unexpected stories from the busy star.
“There was a period of time when huge sunglasses became my trademark, but that didn’t start as a fashion statement. I started wearing them because I wanted to hide my face as much as possible. Because of work, I had to change hair and make-up styles several times in one day, so the condition of my skin and hair deteriorated. The sudden change to short hair was also a last resort due to that.
|
-from Misa-chan, Numero Tokyo
Basically, it's been 'thinner' since like 2001.