I'm gonna chime in here, since I worked directly on these helmets, though not the Star Corps variant.
The CloneTroopers seen throughout Episode III, we're all scaled at different heights. Yes, silly I know since they're all supposed to be, well, clones, but that's the way it was. Dramatic licence supercedes logic in any movie and the Clones were cheated depending on what environment/characters they were being seen in/or next to.
MR were provided with a list of the battalions and their associated heights. Some were scaled at 5'10" some at 6' etc. Temura Morrison is not a tall chap. He really isn't. He's shorter than Ewan, who himself is not a tall guy. Don't believe me? then check out the scenes in Jango Fett's apt. in Episode II and you'll see that Temura is shorter than Ewan in all the long shots except for the close-ups where they cheat the character's eyelines - another common practice in movies.
When ILM scaled the CloneTroopers larger they didn't rebuild the data files they just scaled them up percentage wise, so everything was proportionately bigger by 2% to 3%.
The MR CloneTrooper Helmet is NOT smaller than any of the ones seen on screen. If anything, it is physically larger than the smallest screen seen variant, though exactly the same proportionately.
The MR Clone Trooper Helmet is made from the EXACT digital data files supplied by LFL from ILM.
MR have already explained the paint job on the Star Corps helmet was dictated by LFL supplied reference.
Do things change from pre-production, through production, to post production? Hell yeah they do. As a licensee, what counts is what LFL supplies. They call the shots on product.
The General Grievous Cycle toy from Hasbro is a mirror image of the one seen on screen in the movie. Why? Because it was originally designed that way, but later changed at some point during production. In a CGI world where you can change every detail of every single frame and you're George Lucas who likes to finese everything as much as possible before he locks the final print, expect changes.
All the above aside. I think these helmets are incredible. A lot of good people worked long and hard to make them, in my opinion, the best replicas of Episode III CloneTrooper helmets ever offered
Barry