You get black bars if the aspect ratio of your source is not the same as the output aspect ratio (which matches the video player aspect ratio of Youtube). While cropping you change aspect ratio, if the x-cropping size is different to the y-cropping size. Size means the percentage of the width and of the height, not pixels. For example, if you x-crop 100 pixels from the width of a 1920 screen, this is 5.2 percent. To keep the aspect ratio, your y-crop has to be 5.2 * 1080 / 100 = 56 pixels at a screen height of 1080.
Crop the canvas in a way that aspect ratio is kept the same. Then you can scale up the cropped source without getting black bars.
You can also not maximize the app. Instead, you change the window size in a way that the cropped version of the canvas gets the 16:9 aspect ratio. Then you can scale up the cropped source without getting black bars.