Check that the application is only installed once in these directories:
/data/app/ for user-installed app directory
/system/app/ for pre-installed system app directory
If an app comes preinstalled with the phone or your custom ROM ships with it, then the original .apk will be installed in
/system/app/ (which is read-only, unless you root your phone and remount the directory to be writable) so that you can uninstall the updated version of an app or do a Factory Reset to restore all apps to their original version. This can also happen if you have rooted phone and do an installation by copying an apk to
/data/app. In short, updated system apps takes double the space than user-installed apps as the system stores the original .apk as well as the updated .apk in separate directory/partition.
If you are 100% sure that you will not downgrade or do a Factory Reset, then you can root the phone and delete
/system/app/BlahBlah.apk to save some space; although this might not actually give you more space for storing data or applications unless you repartition the internal memory since the
/system/ directory is usually a separate partition from the user-installed app directory. In short, don't do anything unless you really know what you're doing.