Just right (mfg)
Too light
I walked 195 miles from Santarem to Porto carrying a backpack that weighed between 11 and 12 pounds. A few days before we got to Porto I started getting a sharp pain in my neck and shoulders that got intense at the 14 mile mark and didn’t really go away when I put the backpack down.
Waaah waaah waaah. I assumed my body would acclimate, but the pain got worse.
Turns out my shoulders were taking too much of the weight, perhaps because the pack lacked a hip belt. The backpack is actually recommended for weekenders, not ultras. If there is such a thing, the pack was too light.
Too heavy
In Porto we bought a new backpack - pretty heavy, but recommended by the folks at the Decathlon store. It was designed for women, had a hip belt and fit well.
So I walked 17 miles from Porto to Vila do Conde in a pack that was easily 18 pounds.
Though my hips took more of the weight, it was too much. My neck and shoulders really hurt.
Just right
There’s a remarkable service called Top Santiago that picks up your backpack in the morning and delivers it to your sleeping accommodations same day.
I carried my own stuff for slightly more than half the trip and that was enough for me.
My Camino now involves backpack transport by truck.