LONG BUT WORTH READING IF YOU ARE SHOPPING FOR BAGS.
Since someone saw fit to resurrect a 9 month old thread, I will take this opportunity to point out that the Ortlieb Back Roller (Classic or Plus) panniers are capable of being loaded significantly beyond the stated 40l capacity, that relative newcomer Ortlieb City Roller bags are a sweet pannier value, and how Arkel Orca bags are notably heavy and expensive.
Ortlieb Back Roller Classic (OBRC) panniers are 60cm in height total, laid empty on a flat surface. OBRC panniers
achieve 40l in volume when the material is rolled several times down to a height of 42cm. It is possible to overload to a maximum of 52cm in height while still maintaining a two-fold closure, which is more than adequate for complete waterproofing to rain. This increases total volume to at least 52/42 over the specified 40l,
approximately 50l. This volume estimate is actually underrated due the geometry of the lower section of OBRC. Both front and rear panniers have overload capability, but Ortlieb does not mention this fact anywhere - you have to own/use them to know this.
Ortlieb sells a budget version of their Roller Classic panniers, the Roller City. City are basically the same as Classic (both nominal 40l volume for a pair), minus the aforementioned "overload" capability, which explains why all of the specified dimensions are identical except for weight. City bags eliminate the additional material required for overload, resulting in 760g lower weight for a F&R set (2730g vs 3490g). City panniers typically sell for almost HALF the price as their Classic big brother. I just priced a set of F&R bags at modernbike.com (consistent low price on Ortlieb) at $206 vs $380.
In conclusion, if you are satisfied with the nominal volume capacity of Ortlieb Roller Classic front and rear pannier pairs (25/40l respectively), you can reduce pannier weight by 22% and pannier cost by 45% and still have the quality and reliability of the ubiquitous Ortlieb Roller Classic by choosing the relatively unknown economy version, the Ortlieb Roller City bags.
Ortlieb Roller City bags are lighter, less expensive and probably at least as good as Arkel Orca bags, which weigh 4410g for a F&R set and cost $410, making them the heaviest and most expensive set of panniers in this comparative post.