Yes, Daniel, you're right.  If you're only using paper, the weight of adhesives is infinitesimal and the water content evaporates out.
Mind you, 5 pounds of paper is a lot.  For most things, you don't need anything like that.  If it's a large piece, you are more likely to build up the centre with an armature (of wood, foil, wire, polystyrene etc. etc) and use the mash as an outer coating.
If you use filler (plaster, sand, sawdust etc. etc.) this will provide some weight.
Basically, papier mache articles are very light.
David