I'm a guy, but when I worked as a clerk at a hospital ER, I also filled in as the clerk for our Mammogram clinic from time to time. I learned a little there.

My understanding is that what matters are the makeup of the breast in terms of size and fat/muscle relationship and also the skill of the tech.

Some techs are better then others, but my understanding is that no matter how good the tech, sometimes the breast just has to be compressed so much to get a good read that it's almost impossible to make it pain free.

I'm not an expert though, so if anyone has more or better info, feel free to correct me if need be.