I can no longer count on my fingers and toes the number of new gardeners who have told me that compost is a waste of money or time, because it only disappears (a variation : the rain washes it away into the lava). People misinterpret what's going on. As organic material decomposes, it's moisture content is released, thus leaving behind just the organic mass. The moisture in plants is a large part of the volume. The more juicy the material, the greater the loss of volume.
Above is pictured my newest compost pile. Just two weeks ago I had filled it to the very top and capped it with cardboard. I have used plastic coated cardboard boxes which would help retain the compost moisture and heat better. Eventually they will be removed and discarded, thus not be incorporated in with the compost. But in the meantime they will help keep things evenly moist.
In just two weeks the pile has gone down 12 inches. No, I haven't jumped on it. This has happened on its own with no outside help. As the organic material heats up and rots, it loses moisture. Hot moist air rises, so even with the cap, much escapes the pile. Plus, air spaces in the weeds, grass, and other additives disappear as the material collapses and compacts. This is totally normal and to be expected.
By the time I harvest this compost in 3-4 months, I fully expect the pile to be only half of its original volume. Once the compost is added to the garden soil, it will continue to decompose and lose moisture, continuing to also lose volume.
Rest assured that this is normal, to be expected, and actually a good sign that everything is working fine.