HI ShadowGio1994, Two things that I have played around with in order to change the scale, because I believe it is dependent on the mobile style of youtube.

1. adding &app=desktop to the end of the url makes it show up in desktop mode (you can also do this in Firefox from the three dot menu and tap "Request Desktop site")
2. Turning on the screen rotation on the phone, or viewing the video in full screen with the icon on the far right of the player.

I hope this helps, was there anything specific that you noticed also changed fro the way it was before?

Hi ShadowGio1994

I tested the same video on m.youtube.com with my Samsung S6 Edge, Android 6.0.1, Firefox 49 for Android and it worked fine.

If you haven't already, please try just tapping on the "expand to fullscreen 2 arrow" icon next to the speaker icon to make the video full size.

If that doesn't work please let us know which phone or tablet and what version of Android you are using so we can help you troubleshoot further.

p.s. please try the stuff guigs mentioned in her earlier reply as well

Cheers!

...Roland