The auto grow setting on the db has been triggered at some point and the file/s have been extended. Allowing data to be written into the free space. If you shrink it the db will grow again anyway. If you lower the setting it will extend the file more frequently at a cost of performance each time.

Make sure the auto grow is set to an appropriate amount of MB and not %. All dbs should have a decent amount of free space to grow into.

See it like this: Its the same as buying clothes for small children. You never buy clothes that just fits today always buy the size up so they can grow into it.