Back to the question of volumes, for a trip where I am camping and will see weather that will range from warm to freezing and wet to dry, but no plans or equipment for snow:

40 liters - Ortlieb Backrollers
25 liters - Ortlieb Frontrollers
31 liters - Small duffle that sits on top of the back panniers. Ortlieb, Medium but I do not remember model name of this duffle.
7 to 9 liters - Handlebar bag.

If I have recently bought food, the duffle may be nearly full but it often has only about 10 liters of stuff in it. I pack the panniers full, only the duffle varies significantly in volume over time.

If the weather will not be down to freezing, I obviously will need less sleeping bag temperature capacity, less clothing, etc. But, I generally do not go touring if the temperatures are warmer that this range.



I have only done one tour in mid summer where there was no chance of freezing and on that trip the duffle stayed home - only used four panniers and handlebar bag.

I however tour with someone that uses two really big rear panniers, no handlebar bag, no rack pack. But, he packs really light with minimal spares, minimal clothing options, etc. I am not sure but I am guessing his two panniers add up to only 55 liters. But on our last trip he needed a spare bolt from me, his first aid gear was inadequate, once his only pair of shoes got soaked he had no other option, etc. A couple weeks ago he bought another pair of panniers to put on the front.