Independently of what is possible with your epub files: it may simply be that these pictures in the mobi books have a fixed size of 600 x 800 and that's why they don't scale proportionally to the font size and they always fit the screen. Have you tried it on a Kindle DX (9,7") too?
Edit: You will find some ideas how you can change image behaviour in epubs yourself and manually in this thread:
https://www.mobileread.com/forums/sho...d.php?t=205365
But you would have to apply this for every book anew.