Yes thats possible, but doesn't makes sense. If the framebuffer is smaller than the LCD you will see black areas where emWin can't write to (because of the too small buffer).
The frame buffer should have at least the size of the display (x-size * y-size * bytes per pixel).
In general this is not possible. In this case you would require one of our other drivers, for example GUIDRV_FlexColor.
There are some devices which allow you to write into a specified memory area (using GUIDRV_Lin) which then gets automatically send to the LCD controller via an indirect interface.