The image size is initially set to fit the 3D scale box. It is changed by the Zoom In and Zoom Out toolbar buttons, the + and - keys on the numeric keypad, or by cursor motion with the right mouse button and control key both held down. Holding down the control key and pressing the + and - keypad keys results in a greater change in size.
In the perspective projection, the viewing volume is fixed. Making the image smaller is accomplished by moving the model further from the viewpoint. If it is moved too far away, it intersects the far clipping plane and will eventually be clipped out of existence.
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