I had a sale for 7 credits ( M size ) and I got 1.73 dollars for it. Then had a sale for 8 credits ( L size ) and only got 1.54 dollars. How is it possible to get less money if the picture was bigger and more expensive? 
a) Credit Values depend upon the price the customer paid, so when he/she bought a bigger package, the price per credit will be lower.
b) Your commission varies by image level, from 25% for Level 0 and 1 to 45% for level 5.