The projector has its own projecting resolution as well. You have to access its built-in settings to adjust the display resolution. Most of the projects have an OSD that can be accessed through the buttons. You can adjust the resolution from there. Keep in mind that a VGA connection will be limited to a certain resolution, therefore use HDMI instead.
Additionally, configure the display resolution for the second monitor as well, and the projector will adjust itself based on that.