A smaller buckle, a la Granite Gear, would be nice, but I think a one-way purge valve will be essential. (Also on one of the GG products.)
That way you can fill and seal the bag, stuff if in your pack, and as you load gear on top the air flows out, allowing the bag to compress, and simultaneously flow into all the nooks and crannies in the bottom of your pack.
The best of all possible worlds, IMHO.
Otherwise you're packing a balloon.