North by Northwest was filmed with 'Vistavision', a 35mm Camera which shot using a larger area of the film stock by positioning the film in the gate horizontally rather than vertically. This provided a wider shot without using anamorphic lenses.
I would assume Hitchcock positioned the camera relatively low to place the characters just below the mid point of the shot to give a sense of of scale.
I found this photo which may well be the filming of the shot: The camera is a beast:
This might be of interest: