DPI os an old term used for printing pictures as in a newspaper or books. The graphic was photographed through a halftone screen to break it into various size dots. You can look at an old book with a magnifying glass and see the dots. DPI means Dots Per Inch. It is not used in digital pictures and has nothing to do with size. It is a resolution quality.
The pixel is the digital term. Screen resolution determines the displayed size. A 600 x 800 screen setting is a different size than a 768 x 1024 screen.