Check out this explanation. Basically, the closer the center of mass is to the rear wheel (L), the less force is required to pull a wheelie. Also, as the height of the center of mass increases (H), the required force to raise the front wheel reduces as well. Of course these two need to be considered together, so rider position is still important. A heavy rider will likely increase the height of the center of mass (unless they are some squat machine with no upper body mass), but if they ride further forward, which increases L, the height difference could be cancelled out.
Taking the rider out of the equation, an S will wheelie easier than an Scg just due to the height difference.
Another variable I can see is suspension set up. If you have it really soft, then the bike is going to squat down and effectively reduce H, and increase the required force needed.
Wheelie calculations