I carried a small sleeping bag the first half of the camino (a 40F that squishes down really small), then traded it in for a liner halfway. Both fit just fine in the bottom of the bag, with clothing rolled up on top that, towel in its own outer pocket, etc. I think the key was rolling up things and compartmentalizing them, just fitting everything in around everything else. I clipped my sandals and jacket to the outside of the pack with a carabiner when I wasn´t using them. Three shirts, two pairs of pants, three sox, a poncho, sleep sack, bandana, hat, towel, toiletries, maps and diary. Other small & needful things went into a small pouch/handbag worn out front. It worked just fine, and as time went on and I shed more and more stuff I had room to spare.
Reb.