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The linux software attempts to scale the screen but how this is handled between different aspect ratios I don't know. You can try turning off (or adjusting) the scaling on the linux side.

The vnc command is launched from the file:-

/etc/xdg/openbox/autostart.sh

The line reads:-
vncviewer -listen -scale 1024x768 -fullscreen -16bpp -nobs -graball &

Maybe play with the scale setting or turn it off.