As far as you know weapon weight only works in weapons that you're currently using while all weapons in your holsters doesn't contribute any weight. You have a CSBNG which is pretty much heavy and a pistol when youre holding your CSBNG you can't move fast but when youre holding a pistol you could move fast even though CSBNG is in your holsters.
Weight Stacking will become a way to stop weapon hoarding and to make player become much more specific to their weapons plus it would become much more realistic since even heavy guns in your holsters could still affect you weight.