Decided to compare the size for one of my manga volumes (Skip Beat Volume 41).
https://www.amazon.co.jp/dp/B076F6WTG3/
https://www.amazon.com/dp/B07FR13YYV/- website: 118,847 KB (my guess: 1280x1920 res)
- azw3 download: 38,755 KB (variable resolution: 1280x1920 max, ~220 KB per image)
- PW3 kfx download: 35,180 KB (assets folder 35,028 KB, 965x1448, ~180 KB per image)
comiXology
- in-app, proprietary format: 384.6 MB
https://www.kobo.com/us/en/ebook/skip-beat-vol-41- epub download: 151,273 KB (1400x2100, ~880 KB per image)
At 33,071 KB, that's even smaller than the PW3 download (designed for ~3.6" x ~4.75" screen) so I'm not at all surprised if Kanji is barely legible.
I don't actually read manga via the Kindle app. In the US, a lot of Kindle comics on Amazon are available as "Kindle & comiXology" so I use the far superior comiXology app for reading Amazon-purchased comics and manga. Granted, yeah, it's pretty killer on storage.