Interchangable backstraps and side grips are nice, but they don't change the most inportant dimension, the circumfrence at the top of the grip where the web of the hand (between the thumb and forefinger) meets the gun. This dimension direclty determines the trigger reach.
The Sig Grip Modules vary this dimension.
I have never been comfortable with any service caliber double stack handgun. The worst example for me was the Glock 26. I can't my small hand around the grip to adequately control the handgun. The best I had found was the XD-9 Tactical 9mm. The P320 Compact, with the Medium grip module was a bit of an improvement for me, but the firearm transformed when I got the Compact Small grip.
Even Though I don't need it, I also bought the Compact large grip module. At the range I can swap out grip frames in under 30 seconds, for shooters with different hand sizes.
Also, the exchangeable grip modules make cleaning the gun super easy. Simply field steip and pull the Fire Control Group, spray it down with cleaner and brush clean.