It’ll break in a bit and conform to your head at least mine feels that way after wearing it for a while (also an SK-6 owner). If the helmet is causing you discomfort after wearing it around for a longer period say half an hour then you probably need a bigger size although maybe someone else can chime in especially those that have more experience.
victor at gphelmets told me some of his pro drivers actually put on the new helmet and watch and entire movie to get a good idea on fit and help break it in.
I wear a 7 1/4 baseball hat and I use a Medium GP-5 helmet. They do fit more snug then most helmets, but once they form to your head, you will love it.
I have a friend who wears a medium, but he went with a size up cheek pads, because they are smaller. The helmet was too tight around his cheeks.
I hope this helps.