[Closed] specialized ambush helmet sizing?

Tried a spesh ambush helmet on in the shop at lunchtime, seemed a really good helmet lots of coverage, quite light for the coverage it has, and really liked the way the winder fits into the back of the helmet. They only had a large to try on, and although I was able to snug it up nicely, I seemed to take up loads of slack with the winder enough to maybe consider that a medium might fit. Normally I would take a large in all helmets, and even some of them might still be a bit small.

Anyone have experience of spesh hemet sizing, especially say comparing a ambush medium with a mavic notch large?