Why is Mobi bigger than EPUB?

Related questions

1. How do I convert MOBI to EPUB?
2. What is epub Mobi?
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4. How do I convert ePub to Mobi?
5. Can Acrobat convert PDF to EPUB?
6. Is Kindle EPUB or Mobi?
7. What can open EPUB?
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14. #larger
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17. #source
18. #within