I suggest the "popcorn" rating for sporrans. The site selling a sporran fills it with un-popped popcorn and then publishes the weight of the popcorn the sporran can hold. The second question would be can a softball easily fit into the opening of the sporran. If a softball will fit in the opening then most hands, with something in it, will also fit.

Low cost test and if everyone uses it it would be comparable.