If the fullscreen resolution of your game is smaller than the resolution of your OBS settings->Video->Canvas resolution, the capture source only occupies as much space as it is according to the game resolution. You can either:
- set the game resolution the same as the canvas resolution (this records the game with a higher quality, but produces higher system load)
or
- reduce the canvas and output resolution to the game resolution (this records the game with the current quality, but without blank space in the video)
or
- fit (rescale) the game source into the current canvas by right-clicking your game capture source->Transform->Fit to screen (this records the game with a higher resolution but with the quality of the lower resolution: this is the worst of the three choices)