At the risk of sounding like a demon 'less is more' type - I really would not have enjoyed my introduction to the Camino in May with anything more than a small and light pack. It took seemingly endless packing and re-packing, weighing and disgarding unnecessary stuff - but I really didn't find there was anything I missed - and we encountered cold nights, hot days and even torrential rain! My pack, a waterproof liner for it, and walking poles were the biggest expenditure - everything else was from charity shops or homemade.
I took a Berghaus Freeflow III 20 litre pack (£32 on the internet) and the final pack weighed just under 6.5Kg without water. It allows airflow between you and the pack (heaven when you can catch a breeze to cool you down), and although I have a small frame, it fitted perfectly. But even this amount of weight really makes its presence known on a tough day - and I would wince at the grim-faced folk heaving larger packs on after a break.
But as everyone has said - shop at quiet times if possible (I tried mine out in Millets, but bought online because I was picky about the colour - sorry Millets!), and if possible take a heavy bag of shopping with you to check what it is like with weight in it.
Good luck!
Pip