@Nimade lol:]. That's not an issue. You simply enabled "Cardboard" feature. Check this to understand what this feature does basically allows you to use your phone as a cheap VR(in ppsspp case it's 2D). Disable that feature and the game should fill the full screen again.
http://forums.ppsspp.org/showthread.php?tid=6594 - Custom PPSSPP Shaders!
http://forums.ppsspp.org/showthread.php?tid=3590&pid=117172#pid117172 - simple CE scripts to help creating CWCheats,
https://github.com/LunaMoo/PPSSPP_workarounds - CWCheat workarounds.
http://forums.ppsspp.org/showthread.php?tid=3590&pid=117172#pid117172 - simple CE scripts to help creating CWCheats,
https://github.com/LunaMoo/PPSSPP_workarounds - CWCheat workarounds.