This is pretty weird... Do both of them use the same screen height?
I cannot reproduce this reliably. Sometimes Chrome shows the same as your Chromium (smaller) and sometimes Chromium shows the same as your Chrome (bigger).
Oh, yes, it is almost definitely the screen height. Moving the window to a screen without a taskbar shows the bigger video and vice versa. My resolution is 1920x1080.
There is a difference between Chrome and Chromium, though - Chrome supports H.264 while Chromium does not - but this seems irrelevant in this case.