> My car is around 20 cubic meters,
Not unless you drive a medium sized truck (not pickup). Assuming your "truck" is 2m wide, you have a 5m long chassis and 2m high ceiling in your truck. That's enough to haul 20 cows or so around comfortably.
A normal car is about 1.6m wide, 4-5m long, although usually 2 of those are the hood and tail, and not more than 1.5m high in most places.
Evening that out a bit, and your car only is about what, ~8m3.
Most car loads are under 1m3, people highly overestimate the storage size a car can fit in their trunks, even SUV's only store about 1.5m3.
Three car loads of that, and you're still under 5m3.
You're completely overestimating sizes. Your hamper is probably more like 0.1m3. Really, 100l is big for a hamper.