I'll throw a bit of info out here.
I actually built the helmet in question a couple weeks back, and the printer re-sized it as you mentioned above, way too small to even come close to beign accurate.
In the end, I had to just scrap the project, because the way the pep file was unfolded didn't allow for some of the parts to fit on the paper correctly.
That might be my lack of knowledge with the pepakura program, but even when I fiddled with the scale and such, it still wasn't able to fit.
Not that this is anything you haven't found out about already.
My advice would be to give another of the Guy helmet pepakura models a try.