This waterproof features an elastic shoulder panel for stretch fit while the back panel is pleated to expand over back packs.
Wearing it over a bag improves the average breathability slightly as the back yoke flap still works (although you lose the shirt-to-shell direct wicking effect). The jacket is cut large so we'd advise trying a size down for a snug seal at neck and wrists.
The diagonal asymmetric zip is smooth in use, even when filthy, and the mesh neck and one-handed hem drawstring stop draughts. The chest logo hides a port for a drink pipe and there's a zipped pocket on the other side for keys, etc. Weight is impressive at 248g but you've got to really like the bag features and brand for the price.