MongoDB is a datastore which is often used when storing big blobs of data. It has the characteristic to not release diskspace when a lot of data is removed. When you have 5GB of data stored and try to remove 2GB of that, MongoDB will not release that 2GB of diskspace. The disk space will stay at 5GB. Either compact or repairDatabase can be used.

compact

To actually reduce the disk space of a MongoDB database the compact command can be executed. Running the compact command places a block on all other operations at the database level. The compact command works at a collection level. To release diskspace previously used for myMassiveData collection, the following command can be executed:

db.runCommand({compact:'myMassiveData'})

MMAPv1

Using the compact command only works if you are not using the MMAPv1 engine. To release diskspace for a MongoDB instance using MMAPv1, the command repairDatabase can be executed on a database level. Warning: repairDatabase requires double the database size with an extra 2GB.

db.repairDatabase()