I am going by what Don Bies himself posted at the RPF.
I edited his posts together but he is Artoo77, it was a full 7 years ago when he posted this
http://www.therpf.com/showthread.php?t=65622&page=6&p=919620#post919620

"Ivo Coveny, the head of costume props in Sydney, made the decision, I assume along with Trisha Biggar, to remake the mask. All this trash talk about the costume being smaller is wrong, as they had made a muscle suit for Hayden and put lifts on him and everything…if it was smaller, it’s not by much, and I don’t think the helmet and mask are any smaller, IMHO. And I stand by another statement I made in a different thread that it made no difference that Prowse wasn’t in it, as they could have gotten anybody large enough to fit in the costume to play it…and little known fact, there are two other ILM employees playing Vader in the creation scene.

It is my firm belief that it was changed just so that they could call it their own…any talk of “customizing it for Hayden” was just covering that fact.

And BTW, I like that mask too.

It was created using that 3D scanning machine by Ivo Coveny and his team in Sydney, Australia. They recreated the mask, painted it, assembled it and filmed it at Fox Studios in Sydney using Hayden Christensen. Much later, the molds were sent to ILM and we made the three part (helmet plus split face mask) Vader for the shots involving the mask being assembled onto Hayden’s face (who wasn’t present, as they had shot his scenes while still in Sydney without the mask coming down onto him). Additionally, we re-shot at ILM some of the sequence with the table rotating into position, as Hayden was originally filmed with his hands up near his head, and George (I assume) decided that he should have his hands at his sides." --Don