I have a 56cm head and just received my M size prolight helmet.
The shell itself is too large, the helmet feels loose on the sides and back.
Therefore, it hurts the top of my head when the straps are correctly tightened.
Just purchased a S size to check if has a more snug fit on the sides.
Anyway, it's really light and i liked the elastic strap system on the back - much more comfortable then the regular systems.