Any digital image, whether scanned or otherwise, is an arrangement of small square blocks or dots called pixels. The smaller the pixel, the finer the image; but the total number of pixels is greater.
dpi = dots per inch. The higher the dpi you use to scan an image, the more bits of information it records, resulting in a larger file size.
To be more precise : Suppose you scan an image of 2"x2" size. At 100dpi, you have 200 dots across and 200 dots down, and you get 200x200 = 40,000 dots. Like this -
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If you scan the same picture at 600 dpi, it will have 1200x1200 dots, a total of 1,440,000 dots.
To be meaningful, each dot has to have its own shade of colour. If you scan at 24-bit colour, each dot has its colour information stored as a 24-bit piece of data. Therefore a 2"x2" picture scanned at 100dpi at 24-bit colour depth is represented by 40,000x24 = 960,000 bits of data.
At 600 dpi, the same picture is represented by 1,440,000x24 = 34,560,000 bits of data.
8 bits = 1 byte.
So the 100 dpi picture occupies 960,000/8 = 120,000 bytes = 120 KB.
The 600 dpi picture size is 34,560,000/8 = 4,320,000 bytes = 4.32 MB.
A 10" by 8" picture scanned at 600dpi and 32-bit colour = 115.2 MB !!
The file size can be reduced by compression techniques, but it results in some loss of quality.
A full explanation will be too long to post here, but I hope you get the idea.
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