I have a 59 cm head, which tends to lie on the border between medium and large sizes for helmets. I tried large Giro Prolight, but was very disappointed with the comfort and fit, and thought it looked big and cheap.
I just got a Kask Vertigo C50, like wot Team Sky use, but in White/Silver, in large. Looks good on my head, and the fit is really great. There isn't much to adjust, and I didn't need to. Turning the dial on the hinged rear fitting tightens it around my head to give a nice snug but comfortable fit. The inside of this hinged section is lined with comfy silicone rubber padding, and the rest of the inside of the helmet has more padding than Giro models. The artificial leather strap under the chin is really comfortable, and they have lined the nylon straps which go past the ears with the same material. It is really nicely detailed. Unlike most helmets, the rim is covered with plastic instead of being naked expanded polystyrene. In general there is very little exposed polystyrene on the outside of the helmet, which should help durability.
My large weighs 315 g, which I think is about average for a large helmet, but probably 100 g heavier than the Giro Prolight. Given the fit and detailing I don't care.
Heads are different shapes, of course, but I'd recommend trying one on. Unfortunately not many places sell them, so you'll probably have to do this via mail order.
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