By default, the ePaper beings in "Page Fill." That allows the ePaper reader to fill up as much of the page as possible. If you'd like to make it bigger you can change to single-page mode by clicking the single-page button in the top navigation bar:
If you'd like to undo the changes to the ePaper, and move them to "Page Fit" mode, which puts the whole page on the screen, you can click the "Page Fit" button on the top navigation bar, which is right next to the single-page button. To change back to "Page Fill" mode, click the same button again.