This was crazy and made me even think what's the meaning of "Compaction" for mongodb ?
> show dbs
db0 35.9365234375GB
local 10.1982421875GB
test (empty)
> use db0
switched to db db0
> db.entries.count()
51927379
> db.entries.runCommand( "compact" )
> show dbs
db0 45.931640625GB
local 10.1982421875GB
test (empty)