It doesn't work the way you think it does. This is by design. Remote connections have ZERO to do with monitors or GPUs on the system. There is no way to mirror the existing desktop programs into a VNC display ... well not without lots and lots of effort using some screen sharing method.
I use x2go (about 3x faster than RDP/VNC and includes ssh tunneling for security) as a remote desktop. The virtual display size can be selected to be almost anything. The real limit is who large is the physical machine you sit at when connecting to the remote system.
With rdesktop ... there are command line parameters which control the desktop size ...
Code:
RES="1280x738"
rdesktop -u thefu -g $RES remote-windows-on-LAN-machine
Sorry that I don't use VNC anymore. Hope that makes sense. I suspect the VNC client should have something similar. RTFM.