In Beth Hensperger's "Not Your Mother's Slow Cooker Cookbook", she suggests increasing cooking time by 1-1/2 to 2 hours when multiplying a recipe intended for a smaller cooker. I have no idea where she came up with that number, as cooking times vary quite a bit, so I would think that a percentage would make more sense. it seems reasonable to assume that it would take longer for a larger pot with more ingredients would take longer to come up to temperature.
I recently replaced a ceramic 4 quart cooker with a Cuisinart 4 quart Multi-Cooker. The MC has a metal bowl, which comes up to temperature much more quickly than a ceramic bowl. I usually cook on the low end of the time range, especially if I start out by browning the meat in the MC.
Beth also points out that slow cookers are designed to be 1/2 to 3/4 full due to the placement of the heating coils.