I'm not even super thick (5.5"-ish around) and this happens to me a lot. It actually hurts some girls i've been with, and my current gf finds it uncomfortable. And yea, it reduces sensation for me too. Best way i've found to prevent it is simply not pulling out too far on the "backstroke". Best way to get rid of it, if you notice it happening before she does, is to pull all the way out and firmly press down just above her pelvic bone... or alternately, you can stick a finger or two inside her and press UP on her upper vaginal wall (same place where the g-spot is). this usually allows any trapped air to escape.