Simplistically, no. For one thing, if the bag moves relativistically, so does the content, so, from the point of view of the load, the bag would have shrunk in the direction of their common axis of relative movement. 

There are problems anyway. What are your contents supposed to be? Lithium? Osmium? Would you not be better off speaking of a one-litre bag being able to hold two litres? If so, then from my point of view, if I happen to be standing to one side of the mouth of the bag I would see the bag just filled for just a fraction of a yoctosecond, whereas anyone passing along with the content would disagree.  he would agree that every part of the load had gone on, but would disagree about when, and in particular, would say that they were not all in together.

At least, that is what it looked like to me last time I tried it.

Or did it?

When things happen so fast, it is so hard to remember which did the moving. I'll have to ask Schroedinger's cat.

Jon