Liz,
@ebrandt sorry did not see your post, mine is the Featherlite which, at 50 litres, plus the balance packs is bigger than needed for a "normal" camino, but very easy to use - no need for surgical packing. I have sports balance packs in the regular length which gives another 10 litres.
If I was walking a summer camino, and paricularly if my back was a problem I would buy the Natural Exhilaration, which is 33 litres. I'd stick with the sports balance packs in regular - not the compact (which only take 6 litres). The idea is to put your small but heavy stuff in the balance packs - so my water, toilet bag (including first aid), phone, charger, camera if I have one. I used to keep my passport, credentiale and wallet there but now they go in my pockets (thank you Maccabi).
There are packs with sizes between the Featherlite and the Natural Exhilaration (Marathon and Mountain Magic) but, having looked at them in my local shop, they don't have the same torso system and hip-belt. I dont really know what they would be like. The Natural Exhilaration is very similar to my beloved Featherlite.
If you are interested you do need to trawl through the Aarn website. The most important thing is to get the right backlength and hipbelt size - the website explains how to measure.
Here's the web page for the Natural Exhilaration and the balance packs
http://www.aarnpacks.com/#!natural-exhilaration/c227d
http://www.aarnpacks.com/#!pivoting-balance-pockets/csr5
Send me a PM if you need any further info. And no, I still have no commercial interest in the company!