[GeoffR]

The most likely(*) problem is that the publisher has used pixel (px) or absolute (in, mm, etc.) sizes in the stylesheet for images, instead iof a screen-relative (%) size. This means that the book will display perfectly on devices with one particular size screen, but images will be too small or too big on all other devices.

Although it is usually quite easy to fix these sorts of publishing problems by editing the stylesheet and/or html files in the book, there are so many different ways the publisher can mess it up that it is hard to describe how to fix it without seeing the relevent parts of the book's stylesheet and html files.

So generally speaking, you need to go through the css and html files in the book to find where the publisher is setting the width and height properties for the images. If they look something like {height:2100px; width:1400px;} then change them to something iike {height:100%; width:auto;} In some cases you might need to change the min-height, max-height,min-width, and max-width properties also.

(*) Edit: Most likely in my experience with ePub novels, but I've never read any manga. There are other things such as margin and padding styles that could cause a similar problem.