[GeoffR]

It is probably because the publisher hasn't specified the size correctly. The size of an image willl be the actual size of the bitmap unless anoher size is specified, although different readers can use other defaults. The publisher should have specified a size in units of em (e.g. 1em) if the bitmap is to be used to replace text characters.

(But really the publisher should not do this at all, as a bitmap is always going to look bad when set next to a scalable glyph. They should really use an embedded font or SVG image instead.)

Edit: If you don't mind editing the book's stylesheet it should be fairly easy to fix, you can probably just change the image size to height:1em;width:auto; for the images that replace text, and height:auto;width:100%; for images that should be scaled to the screen width. But for the bitmaps that replace text it is better to delete the bitmap image altogether and replace it with the actual glyph it is supposed to represent, and then use a font that contains that glyph.