Vince, you're British, right? The current standard Orbit UK paperbacks are small size, except the trades that were sold instead of hardcovers for export (though hardcovers are available in the Netherlands as well). The US Orbit paperbacks are larger size. Do you mean you have a collection of US paperbacks?
I initially bought the US paperback of Against a Dark Background and was horrified to find they had converted the book to American spelling. It went straight into the trash. Might seem strange as I am not a native speaker, but I can't abide that nonsense.