timarelay, I know you've seen my pictures in the "Brain Bag for a small woman" thread. I am using the chest strap to keep the regular shoulder/backpack straps closer together. I see in your picture that your chest strap is positioned fairly low on your chest. I have it a few inches below my collarbone (e.g. just above my um, breasts). It helps a lot with making the backpack straps stay closer together.
I'm guessing some of the difference between the two of us fitting/not fitting the BB is that you're a lot slimmer than I am. :)