I run a W40 and the back pockets are smaller compared to other brands in same sizing (like Levis, Edwin and Nudie). IMO it makes the jeans look weaker when there is more "empty" material on the butt.
A ran into a guy who had W32, albeit in a different model (I have 888s and he had 634s) and I compared the sizing. Identical. Is Iron Heart using the same size back pocket for all their sizes?