I would suggest using ncdu / in the image to show candidates for purging.
E.g.:
/usr/share/doc: 82.3MiB
/usr/share/locale: 63.5MiB
A concrete example for purging would be /usr/share/doc/gcc-4.9-base/test-summaries.
Maybe localepurge could be used to only ship English locales in the default image?
Ref: #30 #37