Lightbox video opening into fullscreen?

Hi, I have a number of videos on my website that open into light boxes. Some of them seem to automatically open as fullscreen whereas some seem to open into a much smaller box (probably about 60% of the screen.

All of the videos are formatted the same, so I’m not sure why this is happening. Is there a way to control the size of the Lightbox display? I would prefer to have all the videos open at fullscreen.

I cannot share the read-only link to the site due to privacy requirements but here are some screenshots for context:

Fullscreen Lightbox:

Smaller size Lightbox:

Any help greatly appreciated!!

We really need something to work with. A duplicate page with no branding on a new site would probably work. Normally I would suspect that the source files are different, HD 720p versus HD 1080p for example. Can’t check from my end. Good luck.