Hi, Janet, and a big, warm welcome to the Forum....
A larger size backpack does not have to be fully loaded.
I would not focus on the size of a backpack to purchase based on this Camino... I would buy the size that would be useful for the majority of the adventures or hikes you will undertake now
and in the future. As an example, if you are considering something like backpacking for a few days time in the future, you need a larger capacity bag, and that is the one I would purchase for this Camino.
Keep in mind, too, that with a variety of manufacturers that now produce quality backpacks which are both tough and lightweight, that one manufacturer's small capacity backpack can actually weigh more than a much larger backpack from a different manufacturer. For example, my Osprey Stratos 24 -- a 24 L pack -- which I use for my daily training hikes, is heavier by 14 ounces than my Gossamer Gear Mariposa, which can expand to a 60 Liter capacity. I have a few backpacks of various sizes, but the Mariposa has been used, and is now being used, on multi-week backpacking wilderness hikes where I need to carry up 25 pounds of gear including up to 10 days worth of food and fuel, in addition to all of my gear specific to wilderness backpacking like a tent, air mattress, and cooking gear.
It is also what I carry on Camino, where I only use about a third of its volume to carry the 8.75 pounds of gear and clothing that I take with my on Camino. The fact that the pack can carry much more than my Osprey Stratos doesn't matter. And the Mariposa also fits the dimensions required for use as a carry-on when flying, so I do not need to check it in as luggage.
Please note that I am
not making a recommendation with my mention of the Mariposa; I am only using it as a real world example.
The capacity or volume of a backpack is not the same as its size, as it pertains to what is a proper fit and size for your frame. The size is usually expressed by the manufacturer as 'S' 'M' 'L' and XL'; sometimes with an adjustable frame backpack, the sizes will be combined to cover that wider range of fit: S/M, M/L, L/XL. This is based on the length of your spine. Below is a video which will help explain this. Ignore any product reference, the information applies regardless of backpack brand or manufacturer.
Please feel free to PM me if I can answer any questions or provide additional help.