This is the reason that helmets of fibreglass and carbon usually have a closer fit. For strength, these helmets had to be quite heavy. However, by combining a thin shell of fibreglass with foam lining, some fibreglass helmets are now among the lightest of all. This gives high stability, good comfort and low weight. But the ventilation is usually not very good.
Plastic helmets are the most common.
They offer good ventilation and comfort. The stability depends on a lot on the harness system inside in the helmet. The weight can vary a great deal. It is the lightest and strongest of all the hard shells, although you have to give more for the luxury. Some ultralight foam models have an extremely thin skin that seams more like a coating than a shell.