App size main storage/sd card

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http://www.mobileread.com – This is on my Kobo Vox but it looks more like an android query I have the Kindle app on my Vox (downloaded from play store and latest version) If as default I leave the app on the main storage it is 18meg If I move the app to the sd card, the size of the app is just over 4meg why is there a difference in size of the app ? data storage for the app wherever it is stored doesn't change, just under 1meg (HowTos)