Two possible reasons (or both!)- The image size of a digital camera is probably (depending on model and the mode set) around 640x480 pixels (dots) and the scanner may be scanning a "6-by-4" picture at 150 dots per inch (dpi), which gives 900 by 600 pixels
Secondly, JPEG is a "lossy" compression, and you can adjust the amount of "loss" that you are prepared to tolerate on your image (for a fast-loading web site I would probably accept more "loss" than for a picture that I was sent by email of family. (if you look at a JPEG with lots of compression, you can see a "contour map" effect on flesh-tones or similar gradiented colours)
Hope that was simple!