Which viewer are you using?
Zooming is possible in both spacedesk Windows 7 viewer and the spacedesk UWP viewer for Windows 10.
In Windows 7, you can zoom in/out using your mouse’s scroll wheel.
In the Windows 10 viewer, make sure to enable the ‘Scale’ option in the settings > display. Once enabled, you can zoom in and out on the spacedesk screen with your mouse’s scroll wheel.
zoom in/out feature will be available soon on Android viewer app.
If the desktop objects appear to be smaller on the Android viewer app,
you may set a smaller resolution in the viewer app.
Just go to Menu -> Settings -> Resolution, check ‘Use custom resolution’ and choose a resolution that fits the aspect ratio of your device. Upon connect, select the new resolution on the Windows Display settings.