Twitch expects a 16:9 resolution stream. You are sending a 16:10. It will columnbox your feed unless you fullscreen it on a 16:10 monitor.
You can fix this by setting your base/custom resolution to a 16:9 res in the OBS settings. It will at least let you control where those black bars are placed, so you can slide your video over to the left or right, and fill the now-unified empty space with something like your chat window, webcam (without blocking any gameplay) or an image asset serving as a 'frame' so it's not just blank black.
Recommended to step up to the next highest 16:9 res, and stretch your source UP (in-aspect ratio) then allow it to downscale from there via the normal dropdown downscale for maximum quality. So if you have a 1680x1050 monitor, set your base res to 1920x1080, stretch your game sources up to fill as much of the preview window as possible while still being in-aspect, then downscale to 720p via the Lanczos dropdown box in the settings. Again, you won't get rid of the columnboxing, but you'll be able to control it at least.
The only way to completely eliminate the columnboxing is to play the game in windowed mode and at a 16:9 resolution and capture that, run your monitor at a lower resolution in a forced 16:9 mode (which WILL distort everything for you), or buy a 16:9 monitor.