Video players puts black bars to avoid stretching. 1680x1050 have a ratio of 16:10.
When I full screen 16:10 ratio video into my 1280x1024 (5:4) monitor, i get black bars at top and bottom, not sides.
When I full screen 16:10 ratio video into my 1920x1080 (16:9) monitor, i get black bars at left and right.
I don't have 16:10 monitor, but running a 16:10 video at full screen 16:10 monitor should avoid any black bars.
Twitch and YouTube video players when not using fullscreen are mostly at 16:9, except for embedded YouTube video with custom size.
Dropbox video player appears to resize depending on if the browser size is wider or taller, so there may be black bars at either left and right, or top and bottom, not both.
EDIT: now looking at the picture at first post, it looks like the video player you used have problem with aspect ratio? Try VLC player, or k-lite codec pack with media player classic.
EDIT2: actually not, as the screenshot resolution is 1680x974? which is not 16:10 ratio. This is probably the bottom task bar claiming part of the screen which then shrink the video.