Hi and welcome to Mobileread.
Sounds like your fonts are using a fixed size, e.g. measured in pixels or actual lengths. Look at your code and see if you have any entires for "font-size" either in you CSS file (if you used one) or in the actual tag in the html/xhtml files. If you do then the units used may be the problem.
Fixed sized units like "px", "pt", "cm" or "mm" will cause the text to be the same size, regardless of the device.
Variable sized units like "em" or "%" will scale with the screen dimensions and zoom levels.
Personally, I don't specify any font size for the base size, and only add it for elements I want to be different, like a title.
If you can't see anything in your code, post a sample up here and we can look at it.