Ahhh yes those margins are definitely a bit tight for 5.1. The packaging process can use anywhere between 1gb and 7gbs of mem at once between memory drops, so it can be a bit rough. Honestly even with 16gbs of my side rig sometimes I feel the draw pretty hard.
I think you could be correct where the virtual memory isn’t being taken into account, but I’m not that well versed in the source especially in the packaging area to be able to verify that.