I think you hit it. I just ran the numbers on this boat since I couldn't sleep this morning and as Toni said (why did I doubt him?) a lighter person is more stable.

I am not certain on the why it is so, but I see two reason that make sense. COG is probably lower. With a lighter weight paddler the righting moment is stronger for the lighter person than the heavier person.

All this is backwards to what I expected. Here is the graph I made on this boat this morning. I used 200lbs to represent Steve and 250 lbs to represent me. I then tossed in a 125 lbs just for kicks. The kid is the most stable of the bunch! Very interesting.