From personal experience having made the transition from backpacking to cycle touring, I often look at the issue in volume. When I was backpacking, carrying a tent, sleeping bag and stove, etc on a long distance route, with most of my food having to be carried in dried format, I always tried to restrict myself to a 70L rucksack with an extra 5L in the pockets.
On my bike I can easily carry the following (and this is what I took to go off around the world)
My front panniers can take 25L (12.L each)
My rear panniers can take 40L (20 L each).
My rack pack* can take 31L
And finally my barbag carried 5L.
Add that all up and you will see that is 101L. I often had space in the front panniers for a couple of loaves of some nice wholemeal (substantial) bread without having to faff around trying to find space for it elsewhere. My rack pack had plenty of room for me to add clothing or the shopping to. Nothing was every full to capacity and that included carrying a lot of medication and 2 spare tyres (because of where we were cycling at the time).
Why do I need any more on a trailer? If I really wanted to I could put the larger (traditionally called rear) panniers on the front as well... and in some areas of the world that may be necessary but I certainly didn't find I needed to be carrying anymore and when I go off on 2-3 week tours, I only carry the rear panniers, rack pack and barbag.
* Ortlieb do an item of luggage called a rack pack that clips into the rear roller panniers they do and is an exceptionally useful bit of kit. It is a long thin bag bag, the width of 2 rear panniers and a pannier rack and rather handy to be honest!