Both your standalone and Web applications have a cool feature: showing the video in original size (pixel for pixel). Of course, when the original is large, it cannot be entirely shown. But which namely rectangle does it occupy?
Zoom has toolbar (at the bottom) and another bar (at the top) which both can be hidden, e.g. with the [Alt] button. This is like the task bar in many desktop environment (e.g. Windows). What Windows does to the space formerly occupied by (hidden) task bar? It offers it for reuse! Likewise, I expect a large picture to fill all the space in Zoom not currently occupied by other elements.
If the space in the bars on the bottom and the top isn’t reusable, then for which benefit do you hide the widgets at all?
The bars become empty worthless black bars.
This is Zoom Meetings 5.8.3.1581 for Windows.