Your dimensions equate to 7.8 cubic yards, Baz.
That's about 6 cubic metres. The measurement is taken at "load level".
Roughly the actual top of the skip.
The load level is for dense, heavy material such as earth or hardcore.
With lighter, mixed waste, it's perfectly permissible to go higher, often using old doors as side risers.
Then, it's a matter of common sense. If it's piled up so high that it's unstable, then drivers often throw some of it off when they pick up.