We shoot quality fisting videos in house and I can tell you a few things that aren't general knowledge.
One thing is that some women have very pliable skin tissue around the anus and it
takes almost NO effort to get a fist in. Other women can hardly get it open no matter
how hard you try - its all genetics
Check out this gallery
This girl Missy is 5ft tall and 90lbs. She took a whole fist in her ass first time without even trying hard!