You should travel as light as possible. Secondly if your back is to big you will have to check it in. Our bags got delayed last year when it was checked it so this year I will use a pack which is small enough as hand luggage. The bag should be between 35 and 40 L and it will force you to only take the minimum. I now have the TRANGO SALINON 35L+5 pack. It is smaller than the prescribed size of hand luggage and it only weighs 1.3 kg. You can get it at Camp and Climb in Centurion. See the link:
http://www.adventurebase.com.au/trango-salinon-35l-5. My Deuter back weighs 3.6 kg!!. It's perfect for my normal hiking and camping but I wanted the smaller bag. Osprey also has smaller bags and you will have to try out a couple. The problem I have is that some of the smaller bags does not have the futures of a proper long distance bag. The TRANGO has all the futures I like and it fits perfectly. I am not using a bladder at all. It is to much trouble to get the bladder back in the bag after filling. There is enough water points on the camino and a 1L bottle worked for me and I attached it with bungee cord to the front straps of the backpack. I don't have to ask someone to hand me my water bottle. T-shirt and gym shorts for sleeping (easy). Leave the silk stuff at home
. You don't need to take apillow - all albergues provide pillows. You might want to take a pillow case but in Spain they use different size pillows then us and you will not find one locally. For the flight take an inflatable pillow and mail it to Ivar when you get there. Leave the yoga mat at home. The mattresses in the Albegues are fine and some will provide you with disposable sheeting and pillow cases.
Remember the corkscrew for the tinto (red wine). Cape Union Mart has the smallest one you get. See the picture below.