It isn't about the size of her...behind, but the distance between her sit bones, both inner and outer. It sounds like the stock saddle is a man's saddle, they are typically too narrow for women. The purpose of the saddle is to support the sit-bones. The first thing is to determine what those measurements are. I need to run out the door so I can't search for the threads for you, but there are many lengthy threads here about how to take those measurements. I am sure that someone will post a link to those threads shortly.

There are a wide variety of saddles for different needs and sit-bone measurements - and it can take some time to find THE saddle. It isn't fully objective, two women with the same measurements and size can take the same saddle and one may find the saddle to be nirvana, while the other may find it a torture device. The measurements do help to narrow the selection, and you should also search for the "cut-out" test, some of us do require a saddle with a cut-out while others cannot ride them at all.