Where do you fit on this chart?
Girth does play a part in this too, as the thicker you are the more there is to get in. Although I agree with LVT to a point, I do think size matters also. I once had a DP session with 2 guys and the one that I knew who was bigger, just couldn't get in the back way, so we had to swap over and the other guy who was thinner in girth had no problem at all.
Having said that being sexually stimulated really helps and maybe the use of poppers would help.
I don't do anal now though