I am primarily a guitar player too (small side of average sized hands) and I am going through the basic mando rhythm videos. I played a little mando years ago, but I just started into the video series a few days ago. One thing to note about how Ben and some others play the closed G, is the angle they are holding the neck. It’s almost like holding a golf club. The neck runs about parallel to the base of the thumb, the thumb is laying next to the neck, and the base of the pinky is almost touching the other side of the neck. If my hand is more open, it’s just about impossible for me to make the chord.
One other thing I noted about making it clean is that you can “cheat” your fingers in a given direction to keep them from muting adjacent strings. For example, on the E string, my finger cheats to the outside of the neck to prevent it from muting the A string. In this particular chord, all of my fingers can cheat away from the middle of the neck and towards the outside.
It’s certainly a tough chord. When practicing chops with it, I can only go for a fairly short time (10-15 minutes) before I start getting cramps. I figure my hand will eventually get stronger and I will in time get a more efficient position, but until then I am just trying to do it a little bit each day.
Best of luck with it. If you find any tips that make it easier, please pass them along. I am not suggesting you give up, but if all else fails, you could always play a G chop in the C shape further up the neck.