[DiapDealer]

Sigil would probably not handle such a monstrosity well at all. "Works for me" is not really evidence that reasonable standards were used to create it.

Are you reading the Kindle version on your Fire tablet, or using an epub with an app? If the former, it's not surprising at all that a binary format, like the various Kindle formats, would perform admirably regardless of size. Individual file sizes are not really relevant to binary Kindlebooks. Kindle apps/readers don't load entire flows (files) into memory like epub rendering engines do.