A jpeg image file achieves a 10 to 1 comperssion. Some details are lost during compression with little perceivable loss in image quality. The smaller file size allows the image to load faster versus other formats like, .tif, .bmp etc. This is the most common file format used in digital cameras today.

Bottom line is, in general a 5MP color pic can average about 15 MB. Jpeg compresses it to about 1.5MB so that you can have more pics on your SD card.

Hope this helps.

Joe
BBO