[Lohman446]

Stopping power is an ill defined term. Energy will be virtually equal at least at the end of the barrel as the two factors involved in energy are mass and velocity. The smaller projectile will, at least in theory, have better sectional density and less air resistance enabling it to maintain velocity longer. The larger projectile will create a bigger hole on impact which may be important but not likely.

In the end I expect all the differences (outside of making arguments of extreme) will be fairly meaningless in terms of stopping power.

Edit: There are some interesting things here though. The larger circumference will have more drag in the barrel. I'm not sure how much of a difference its going to make but I would not rule out the idea that it could be substantial. We are getting into an area of physics here that I am passingly familiar with but by no means familiar enough to really calculate it out