The OnSong app itself is about 50 MB, which may be included in what is displayed. The size of your OnSong library is only a part of the data that could be used by the application. There are some cache files that are created and retained but they are usually small. There is also iTunes File Sharing which is a location within the app where files can be transferred back and forth between the app and iTunes. This is most likely what is taking up space.
Go into the Songs menu and choose the All Songs tab. Tap on the + button and then choose Import > iTunes File Sharing. See if there are files in there that you no longer need. You can then swipe right to leftover each file that you don't want and tap on the Delete button to remove the file. This is used as a location to import and export files to and from your OnSong library.
If this area contains backup files, you will also want to check your backup settings. Go into the Utilities menu (gear icon) and choose Backup. Make sure that the "Append a timestamp to the filename" option is disabled. If not, every time that OnSong creates a backup file it will create a new file. You will also want to choose Dropbox as the backup location so the backup file is sure to be stored off of the device: http://onsongapp.com/docs/interface/menubar/utilities-menu/backup/