You are suggesting using a traditional shooting grip with your trigger hand, and then gripping an extended magazine with your other? With a Glock?
I guess I'm a little thick, but I fail to see how that kind of ridiculous shooting grip, or Glocks in general would be relevant to the Sig 365 discussion. If all you wanted to say was people with larger hands may find a larger gun to be a better choice, I get it, but the rest of your post sounds bizarre. Can you post a picture of the shooting grip you are trying to describe?
I like stick magazines for range use. I'm right handed, therefore right hand on pistol grip left hand holds onto long magazine "tube". Not hard concept to visualize.