Because it is a bag
--------------large enough to contain curtain rods
and
--------------large enough to contain a dissassembled Carcano!

Sorry, still don't follow.
Btw I don't think there was a broken down rifle in the bag Oswald carried simply because I seriously doubt, for a number of reasons, there actually was a rifle belonging to Oswald stored in Ruth Paine's garage.
Also, Oswald is supposed to have carried that rifle on public transport several times (the Walker shooting and his trip to New Orleans). He must have used something to conceal the weapon on those occassions. So, why would he have to make a paper bag now when an inconspicuous duffel bag would have sufficed? It makes no sense to me at all, that Oswald would suddenly feel the need to make a paper bag at the TSBD and risk being seen when other, far easier, alternatives would have been available to him.