I managed to access the FAQ page provided by the developers on their website through Google's Cache technology, because there was a question there that referred to this issue. The developers stated that you can zoom an image in both live and playback, but only on the web. There is a possibility this could apply to the Ezviz Studio as well, but as you can see it's only for the images. There is nothing mentioned about the video playback features.
I recommend checking the built-in Help section to see exactly what you can and can't do while using the video streaming mode. I know that for my security system I have to enable the Zoom first by pressing the zooming icon and then I can use the mouse wheel to zoom in or out.
