Okay OP finally delivers, sorry about the wait!
First off I'll start with the measurements. I'll always start with the pics of my RS gen 2, but for reference, you can tell them apart by the color of the interior and finishes of the exterior. The RS is uncolored fiberglass, while the Minute Fett (I have his deluxe offering) is chopped strand matt painted blue, which is accurate to the real prop. The RS helmet is altogether smoother and brighter. The MF helmet has some texture to it and is more of an off-white.
RS:MF Dimensions (cm)
Width, 24.9 : 24.9
Length, 26.2 : 25.5
Circumference, 75.2 : 75.2
Ear Platform, 14.1 : 14.2
In virtually every metric I measured, Minute Fett's helmet came out exactly the same size as the RS gen 2, down to the millimeter. The only real discrepancy in size comes in the length, but a every other metric is consistent, I don't think this is indicative of shrinkage, but rather the visor being less bowed-out on the Minute Fett. Given the main point of Minute Fett's restoration was the re-sculpting of the visor crack, I wouldn't be surprised if this is the result of visor being correctly fit back into place. It would explain why the prop in its current state seems to have less of a visible "flare" as it did during the filming of ESB, but maybe someone with experience restoring one, like Ord Mantell, can better speak to this.
Overall, if size is your concern, it need not be. I don't know how he did it, but Luis must be using some really good none-shrinking materials. The only real difference between the two is the level of surface detail captured in the casts. I took some side-by-side comparison photos a while ago when I took these, but since then I've received Minute Fett's run of metal ears, so I'll take some new ones this week of both helmets fit with their respective ears.
RS Width (Ear to Ear)
24.9~cm
MF Width (Ear to Ear)
24.9~cm
RS Length (Visor to Keyslots)
26.2~cm
MF Length (Visor to Keyslots)
25.5~cm
RS Circumference (Base of Dome)
75.2~cm
MF Circumference (Base of Dome)
75.2~cm
RS Ear Platform Height (Base to Visor Brim)
14.1~cm
MF Ear Platform Height (Base to Visor Brim)
14.2~cm
Weight Comparison (Just for Fun )
RS: 72.6g
MF: 72.2g