That is how the default display for ePubs are in Kobo devices. But there is a way to use the whole screen when reading...
Plug you Kobo to your computer and find the config file (I believe the file name is Kobo Reader.conf). It should be under .kobo\Kobo
Open the config file (make a copy first, and it's better to use Notepad++).
Next, add this line:
Code:
[FeatureSettings]
FullScreenReading=true
This is still a work-in-progress so there are still some bugs. Like the end of the line on the right side might have some letters cut off. Mostly it is buggy with Kepubs.
After saving and safely removing your Kobo, go to the Reading Settings and you'll see a new option that says 'Show headers and footers'. Make sure that it's unchecked.