Video width is calculated by the colorbox script (normally iframes are not responsive things…).
Default width of the video box is 80% width of the screen and default height of the video box is 80% height of the screen. If you look at the video on a large rectangle screen (for example; on a laptop screen), you may see a larger box than video like on the following screenshot (Your video’s aspect ratios are important here);
If you look at the same video on different devices or on different screen resoulutions, you will get a different view like on the following screenshots;
These 80% width and 80% height values are ideal values for all different devices-screen resolutions.
If you want you can change these values at Gallery settings->Colorbox.
For example if you change width value with “60”, video box may looks better on laptop screens, on the other hand it may looks weird on small mobile device screens (It is actually not a big problem because when you click on a video, full screen mode will be activated on most of mobile devices).
As a result, you can use default values or you can play with the values.
For example if you change these both values with “100” you will get a full screen video on all devices…