The point is that if you use two packs with different capacities they will reach full discharge at essentially the same point. The larger capacity pack will discharge faster, even if they do not have the same IR (which is not related to capacity as far as I know). If you connect a 1,000 mAh pack and a 500 mAh pack to the same load they will reach discharge at the same time. Say it is a 1 amp load, it will take 1.5 hours to discharge the packs. This assumes parallel connection to the same load.