I use a Shoei XR1000 helmet for the bike which is a pretty top-end model, the Shoei shape suit me. Different make helmets are different shapes, a med in 1 make and a med in another make may or may not fit you. Its not just down to the sizeing its down to the shape of the helmet and finding 1 that suits you. A helmet should apply a steady pressure all over your head and face with no tight spots, and like Robin said they do mould to your head
just read that back and not sure it made much sense