a brief explanation

Here's a rough drawing that I came up with to explain the difference between Full Frame still photographs and motion picture images, which are after all a succession of still photographs.

NTS - dims in mm

The difference originates from the direction that film travels through the camera's film gate.

There are complications, naturally. And there are many, many aspect ratios, as can be seen here and here. But this directional orientation explains the inherited size difference between (35mm) stills and motion photography.