Hmm, well, the bag is filled with empty space. Space itself is a not substance. it is more like result of observing photons that our brains generate this "space".
When one makes the bag larger in a vacuum, since space is not a substance, it does not react with the bag at all, rather it stays the same (I am talking about space here not Einstein's space-time construct), so when the bag volumn is increased, we do not consider space, as it does not fill up, but rather is a grid. I think of space as a grid, we don't say did grid fill up the bag, but rather on what coordiantes are the bag's atoms at/were.