I agree with you the Jet is a very tight fit to start with however I believe they are designed to be like this and to 'form' to the shape of your head. I think you would have been better off purchasing the XL for your size of head, this was recommended to me by the retailer I purchased mine from (openface hemets UK)
I have had the jet for the past 6 months and with regular use it is much more comfortable than when I first started using it so the above would seem true.
The difference between Jet and classic Jet?...well, not quite sure but see
http://www.davida.co.uk/type.php?id=classic-Jet
Hope this helps out a little.
Dave