The answer is very simple. The firmware you are using in your Astak EZReader Pocket Pro has a bug. To be honest, it's time for an upgrade. That's a rather serious bug and since Astak is no longer making readers or supporting them, you'll never get it fixed.
Actually, it's a bug in the ebooks--they have margins measured in ems, so whenever you expand the font size, you expand the size of an em, hence you expand the margins. ADE is just following orders.
Also, because most, if not all, of the software (except ADE) in the Astak firmware is open source, there will always be updates/alternatives to the firmware by independent parties, as long as such parties are interested.
If I were to get a new ebook reader, where can I find one that A) reads epubs; B) doesn't have wireless capabilities; C) doesn't have touch screen (i.e., uses tactile buttons); D) doesn't require proprietary software to copy books to the device or update its firmware; E) is available in the US? I have yet to find such a device, or at least one still in production.