Here is an example of a problem in one of my books that might be similar to the problem with your small equations. The text has a word in cyrillic script, but instead of doing it the proper way and embedding a subset of the appropriate font, the publisher instead just embedded a tiny bitmap image of the word:
Code:
<p>... a stack of boxes marked <img alt="image" src="images/Art_P104.jpg"/>, smiled.</p>
As it is, the bitmap will not change size when the font size changes, so it will only look OK at one particular (tiny) font setting, which might be the default font on an old low resolution device.
By adding a relative height (style="height:1em;") to the code above I can get the image to change to match the font size of the surrounding text, which fixes one problem, but it still looks awful because the original bitmap is of such low resolution.