I'm a chubby thigh guy and had a chafing problem with my old saddle bag - ruined two pairs of expensive biking shorts.
The Topeak Aero Wedge Bob Ross showed had enough storage volume (I think I have the medium one) and I when I unzipped the expansion zipper in the middle, the increased length decreased the width of the bag. That seemed to solve my chafing problem.
Until I sold my old hybrid bike and bought a Jamis Escapade and also used that bag on the Jamis. But (shortening a long sad story with more ruined shorts and a broken carbon seat post on the Jamis), it turned out when the bike post slipped down a bit my chubby thigh was rubbing against the badly designed (essentially square edges) seat clamp on and doing the damage. New steel seatpost, correct seat height - no rubbing on the Topeak bag.