While different sized bags are common in hiking, Ergon is the only bike bag we know of that comes in both large and small sizes. A ladder system then raises or lowers the upper back panel and articulated shoulder harness. There’s a bendable metal bar to adjust curve and let air circulate between the four big back pads for a reasonably dry back.
The Hydrapak bladder gets a detachable hose with adequate flow and the zip lock top makes it easy to clean.
It does take a lot of adjustment to stop increasingly obvious top-of-pelvis pressure points when heavily loaded. The stiff fit and unbaffled reservoir mean it can lurch about irritatingly on steep, bumpy, hard braking descents unless you cinch it up super tight. There’s only one relatively awkward to reach hip pocket on the outside too so it’ll be coming on and off a lot to get to the side loading outer compartment or main top loading compartment.
A separate rain cover and clip-on bungee cord are provided but the latter gets in the way of the zip opening. There’s no key clip anywhere, which is disappointing given the level of fit detailing.