I'm not so sure about Anya (EC) but as far as Buffy and Willow, I think the writers had something specific in mind as to the "growing up" effect on each. Esp. Willow. In S-1 and S-2, she was very "girly" in her hairstyle. Very "teenagery". And the more the Willow character grew & matured, so did her hair.
Same with Buffy. Very specific in S-6, when she had her hair cut - or rather, she CUT her own hair, after Spike said how much he loved her "golden locks" - of course, I don't think that was really SMG's hair... it was a wig... I think... or was it? If it was really her real hair, then it sure did grow FAST, because it was much longer in S-7.
Then again, never underestimate Hollywood & their hair extentions! 😀