Its about dpi, inch square or just inch?

DPI stands for DOTS PER INCH in the X-Y coordinates of a plane of the paper. If you double the size, you quadruple the AREA. For the 300 dots in the original X-Y plain, it goes from 300 per inch to 150 per inch, so 300 dots now cover 2 inches from the 1 inch of before. Think a balloon here, on which you draw a square 1 inch by one inch and then blowing up the balloon until the sides of the square are 2 inches long. The dots in the original inch are now bigger and spread over a larger area.