I usually have one good smaller one and if everything's going well, one or two huge ones after. I actually don't like having those little fast ones because I know then the great one won't arrive-little NG kinda gives out.
In trying to explain how it gets to that point, the closest I could get in describing was, it's like a building itch...
I also think it's highly psychological.(Our primary sex organ is the brain, after all). Those small annoying O's are more when I'm self-conscious, tired or not into what I'm doing.