If you look at the different photos in that recipe, you can see that the one on the tree has several times more water than the one purchased from the store. So which '16 oz' is it even referring to?
I'm just going from memory. The last time I found them I harvested an entire tree worth, and that was enough to feed two people for two nights. I think I filled up a Baggu and a half worth, and each of those holds at least 15 lbs of mushrooms.