How does Output Quantity on Co-Product Setup affect the cost simulation?

I can't find anywhere in the documentation that Output Quantity of the Co-Products has impact on the Co-Product cost calculation (R30812).  The documentation indicates that only the Feature Cost % is used in the calculations.  While testing our set up we noticed that changing the output quantity changed the cost calculation of the co-product items.  Is there information somewhere that explains how the output quantity impacts the cost calculations?