To be fair, 6,000 rounds sounds like a lot more than it really is. If I didn’t know better I’d imagine a small warehouse full.
1,000 rounds of .45 cal ammo weighs 49 pounds. The package is not that large, it is the weight. .22 Caliber will weigh less, but still quite heavy.
You can fit 1000 rounds of .223 on stripper clips in a .50cal ammo can. Pistol ammo should pack at least to that density, shotgun shells, not so much...
Regards,
GtG