i think that i know why: the game is 832x448 which when you multiply it by 2, it will turn as 1664x896, hence why it does have a gap when using F11 if you want to go as 1920x1080, you will probably need to use 960x540 be aware there will have a gap on the height which it's about 46px for the top and bottom. but i suppose it's probably not what you attended with the original size of your sprites...by the way, doing math with HD resolution can be a bitch sometime...
edit: i think i know why, it might be in the conf file which your game have the resolution of 832x448.