To make gold leaf, a piece of gold the size of a US 50 cent piece is rolled and flattened multiple times until it is so thin it can cover about an acre of land. When gold is that thin, it does not take much to cover the whole statue.
Unfortunately, a covering so thin is naturally delicate. The statues are placed in the most extreme of weathers and exposed to air pollution of all kinds. For the smaller temples, the statues will be replaced with another of the same model every 15 or so years. The old statue will be refurbished and sent on to another temple in a kind of Moroni statue rotation.
On the larger temples and on the non-fiberglass statues, the statue will often be refurbished in place. This will frequently coincide with a cleaning and refurbishing of the temple exterior as well.