Probably because of the zipper on the Selma. With a zipper, you can stuff more things into the bag than if you didn't have the zipper. I imagine with the e/w Hamilton that the open top prevents that.In theory the Hamilton is larger, but my sister in law sure stuffs more items in her medium Selma than I can get in a east/west Hamilton. I have 2, one of which I haven't used, primarily because of how small it is.