M3u8 is not fullscreen when viewing directly - but is when embedded

I'm looking to implement a m3u8 live stream into an app so I can cast it to my chromecast. The stream is available online. Problem is, when viewing it in the browser, it is possible to press fullscreen and the video stretches accordingly, but this does not happen when I track the m3u8 file with fiddler and play it on a chromecast or in an online player.

For example: Video stretches perfectly, no black border

When tracking the requests made when opening the link, you can see that this is the original m3u8 file:

File: Link

When putting the 2nd link now into an online player such as Hls Player or casting it to chromecast using the Google API, there is a 'border' on both sides (aspect ratio is wrong I think).

Is there any way to fix this?

I've googled how to set aspect ratio/video size in Java when casting but without success.

Thanks a lot!