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I have one of the retroreflective Carnacs at the moment that's great (other than the roadie looks) and I actually ordered another one in black a month or so ago. Different model but the same large-extra large 58-61cm size and it was visibly much smaller than the one I have and wouldn't even go on my head.
I contacted PX and they said they'd changed supplier for the helmets and that was why the difference in sizing.
The dial on my current one is always done right up so I must at the smaller end of the size range but the new ones are just a different shape or something.
Carnac whenever PX have them on offer at £15 or less.