British MK6 Kevlar helmet sizing?

[bolo]

are the British MK6 Kevlar helmets slightly over sized? I normally wear a US hat size 7 5/8 [large] and tried on a British MK6 size medium and it fit

usually anything size medium wont fit, are they made a little bigger?

are they made to wear a cold weather cap under the helmet

[Seigfried]

are the British MK6 Kevlar helmets slightly over sized? I normally wear a US hat size 7 5/8 [large] and tried on a British MK6 size medium and it fit

usually anything size medium wont fit, are they made a little bigger?

are they made to wear a cold weather cap under the helmet

They are made to be worn directly on the head, although they should be able to accommodate a respirator and/or hood of a CBRN suit or headset. There is a green nylon web adjustable strap on the inside beneath the black pads. Is there not a slight discrepancy between UK/US sizes though?