By default, the ePaper begins in "Page Fill" mode. This 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 found on the top navigation bar.
If you'd like to undo the changes and move it back to "Page Fit" mode (putting the entire page back on the screen), click the "Page Fit" button on the top navigation bar (to the left of the single-page button). You can also toggle between "Page Fit" and "Page Fill" mode by clicking this button.