I found this article in the NY Times called "Ouch! My Bag is Killing Me." It’s about how massage therapists and chiropractors are seeing all kinds of injuries caused by giant bags and purses. The purses are so heavy they are straining the necks and shoulders of their owners and causing headaches.
My theory on purses is that you will fill up the space you are given. If I ever carry a bigger purse, I end up finding all kinds of things to carry in it… and when I switch to a smaller one, it somehow all works out just fine. I have been using a smaller "barrel" style purse for a while, and it holds everything I truly need. If you are having "purse pain," my suggestion is to force yourself to live with a smaller one for 3 days, carrying only the bare essentials, and see what you miss. Also, organizing your purse is absolutely key to keeping it from getting too heavy. You can clean out your purse almost any time, while waiting at a doctor’s office or in a carpool line– it’s one of those easy 5 minute things you can do. That goes for briefcases, diaper bags, and laptop cases too!