Why can't I expand the size of the font? 

In the vast majority of books, you can enlarge the text by pressing the small icon at the bottom of the page to choose your own settings for the book.

However, there may be books where the text settings are locked - so called "Fixed Layout".

These are typically books with many images where the publisher has chosen to lock the size of the image and text so that you cannot edit it.

In this case, you can try flipping the screen or reading on a device with a larger screen.