I currently have a s/m helmet and looking at a few others they are one size. It got me thinking about what makes a helmet a certain size? Is it the shell or the band/strap inside the shell? Does a s/m helmet have the same shell as a m/l or is the shell bigger on the bigger size?

I think it’s the headband/suspension setup, yes…

Circumference of the head that will fit in the cradle though always good to remember S/M/L are letters and not measurements, so all good ones will publish the measurement that they designate to the letters.

It depends on the manufacturer.

Some will produce a single shell size and alter sizes with padding/cradles. Others will use differing shell sizes to get closer to a ‘fit’, though still use differing pad sets.

It can look a bit mad having an xl shell on someone with a pinhead.. 😁

Some will produce a single shell size and alter sizes with padding/cradles. Others will use differing shell sizes to get closer to a ‘fit’, though still use differing pad sets.

Interesting as being at the L/XL end of thing and a mate being XXL some stuff just sits on top of my head!! Not found many that will get big enough, some FF wont even go over the chin bar

A ecent manufacturer will have at least 3 shell sizes and some have 5.  any helmet that you have to tighten a cradle to get it to “fit” does not actually fit and is pretty ineffective – TRL research
Its the shell that is sized.  It should be a measurement like 56cm which is the circumference of your head.  I have a medium helmet ( 56cm IIRC) which has the cradle fully out and it fits perfectly