Ahha! Rob, that's why green compound on a solid substratum (I use hardwood, George uses MDF, and you use pine/basswood) gets better the more it is used.the larger particle are friable, and will either dull or break down into smaller sharper particles. On a soft substrate, the larger particle will bed deeper, lessening the their apparent size difference.
I wonder how this compares with compound on leather?
Regards from Perth
Derek