It's a mystery, innit?
We have been putting stuff into ours since the autumn and it never seems to get any fuller. Or at least, when you put a load of grass cuttings in there you think it's going to fill up but you go back a few days later and it's all gone.
I have a few theories.
One is that the compost bin is actually the concealed entrance to a tunnel.
Another is that there are organised gangs going round and stealing our vegetable matter.
It surely can't compress down that much?