I shot a video on my iPhone 6s in portrait mode and uploaded it to our Media Gallery. Though playback on the iPhone appears as it should, desktop playback is 100% width and landscape mode (without bars to maintain the aspect ratio). As always, I attempted to reproduce the issue here to find out if it was isolated to our environment. The same thing occurred here.
You will notice that if you can get that to open and play directly in Chrome that it is Chrome itself that changes the aspect ratio and scales the video oddly.
By way of contrast, if you open the same in the latest Firefox Developer Edition, it has the correct aspect ratio, there's no scaling and the image is simply pillarboxed:
In fact, if you open the media page up in Firefox, it also works fine:
The good news is, I've also just checked this in Chrome Canary (which is 55 vs 52 which is the current stable release) and the issue seems to be resolved. This is a good indicator that this particular issue is resolved by an upcoming Chrome update.