I ride a 21" frame with maybe 6" of seatpost showing, and my Carradace Nelson sits comfortably on top of my rear rack when using the SQR Uplift - but I think that carries the bag a little higher than a Bagman. So really, that doesn't tell you anything. If you are worried about it, and have a small frame, you could get a Lowsaddle Longflap bag (about the same overall capacity as a Nelson), and use the regular Bagman instead of the expedition.