Why do koalas have to eat such large amount?

Koalas have an unusual fibre-digesting organ called a caecum. Other mammals, including humans, also have a caecum, but the Koala's is very long (200 cms) and it has a blind end. It contains millions of bacteria which break down the fibre into substances which are easier to absorb. Even so, the Koala is still only able to absorb 25 per cent of fibre eaten, hence their need to eat large amounts of leaves.