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Check the page size of the PDF pages.
These should be exactly the same as the physical copy. You will need some PDF editor to change them, or maybe some command line tool can do this. I sometimes discover page size is huge, like 32in x 24in, and conversion can expand whatever the pixel resolution to match. The PDF image objects themselves do not have PPI property, and scale according to the page dimensions.
Kindle Create, for example, can import PDF as image container for comic format. It exhibits exactly this bloating behavior.
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