I think that the first issue in considering which sleeping bag to take is whether or not you can handle down. Many people are allergic to feathers generally, or down specifically. Determine that first.
If you can use a down sleeping bag you have many choices for lightweight bags at all price ranges. I will not go into them, as others have, and I am allergic. However, I have used and can recommend two lightweight synthetic insulation sleeping bags.
On my first Camino in April - May, 2013 I used a 865 gram weight Snugpak "Traveller" sleeping bag. It is the identical bag to the Snugpak "Jungle" sleeping bag, complete to the bug screen, but in red or blue as opposed to the Jungle bag being olive or black. This bag was roomy as it was a semi-rectangular design and comfortable, if not a tad heavy.
This bag is made in the UK but is available worldwide. However, If you can buy it directly from the UK, you should compare the all-in cost, including postage, against the price you would pay locally. In the US, the prices are very inflated. I bought mine from
www.outdoorgb.com and had it shipped to friends in Belgium. They held it until I went over to visit. This saved me nearly 50% overall.
On my second Camino, in April-May 2014, I experimented with a silk liner AND a Cool Max liner nested inside each other. This was very lightweight and very warm. However, it was such a pain to get into and out of as it did not have a full opening, that I ended up donating both items at Roncesvalles after my second night out from St. Jean Pied de Port. Thus, I needed to find another lightweight sleeping bag.
At Roncesvalles, I took the 0920 bus into Pamplona the following morning, as I had previously walked this two-day section, I did not fancy the mud awaiting me, the municipal albergue at Larrasoana, and I needed a new sleeping bag.
At Pamplona, on the way from the bus station to my hostal, I went into the El Corte Ingles department store and found the
Altus Superlight sleeping bag that wayfarer mentions above. It only weighs 650 Grams all-in and is very small. It compresses to about the size of a coconut. I found it very comfortable. It also has a full separating zipper so it can be used as a quilt too.
My only caveat is that the
Altus bag is more of a mummy shape than the Snugpak I originally used. I think that is where the weight savings between the two bags came in. But the
Altus bag costs about €65. So, I was a happy "camper."
I have not yet decided which bag I will use in future Caminos. Even though I prefer to stay mostly in hostals and hotels whenever I can, one never knows when an albergue may be the only bed in town and the bag will be needed. Thus, I continue to recommend bringing one. So, the lightest sleeping bag that works and in which you fit comfortably seems to be the answer.
I hope this helps.