Junk Removal comes with a built-in complication: it is the full-service option, where a crew does the loading instead of you. Ask about this specifically, because it rarely comes up unless you raise it: you are buying time rather than space, so an accurate item list matters more than any size estimate.
Yardage tells you how much room the container has. It tells you nothing about how much of that room you're actually allowed to fill with something heavy. This request comes from homeowners without labor on hand more than any other caller, and the volume they generate is the reason 10 or 20 yard containers are the ones kept in ready supply for it. Know your timeline and know your restrictions. Those two facts get you a straight answer instead of a guess.
This is a space problem, not a weight problem. A container fills to the top with light, bulky items long before it comes near its hauling limit. Ask whether the size recommendation accounts for the material's density, not just its volume -- a driveway full of dirt needs a smaller box than the same space full of moving boxes. Don't split the difference between two sizes. Pick the one that covers the heavier estimate, because running short means a second delivery, not a partial refill.