It's less about their size as more about their flow rate charateristics, especailly at very low duty cycles. Greg does a whole disertation on this but from this feeble brain comes the following.
Ford uses two slopes a Low and High slope to define the non-linear portion of the flow rate for their PCM's. GM doesn'r implement this technique, so Greg has developed the SPA values to be compatible with the GM PCM software.
Hope this helps
Ed M