qt5.9 vnc client can not show full screen of server

I am using embedded linux with qt5.9. I want to control & display my embedded device with remote client using vnc. But client can not show the full screen of embedded device.
 
I start my vnc server using: ./myapp -platform vnc:size=1024x600 . On client side i am using vncviewer or tightvnc on windows and android phone, but neither can see the full screen.
 
I searched a lot and also checked the vnc source code , but i can not find a solution. Please help me. Following are screenshots of my app showing on vnc client(there are also content on the black part of my app, but not showing on vnc client).