Number of pixels
(high quality) compressed
ca. 1.2 MB
ca. 2.4 MB
ca. 4.8 MB
ca. 6.4 MB
ca. 7.2 MB
* Provided that the detail reproduction of the camera matches the theoretical models (excellent lens, etc.)
To illustrate colors correctly you need 3 byte for every pixel: 1 byte for every color red, green and blue.
The JPEG format separates the colors of a picture in its brightness information and two color signals. It reduces the color information and compresses the rest of the information as well as possible and according to the set quality (compression). Data gets lost whereupon the picture quality can range from excellent with no visible loss (even at highest zoom levels) down to low quality where the image is useless.
It is desirable to keep the required storage space as small as possible because too much information requires too much storage space.
When the picture file contains too much information, which is caused by a high number of pixels or by a high setting for picture quality, the following processes slow down. These include the usage of picture editing programs, the transfer of picture data via email or simply loading/opening files.
In summary: the more pixels, the bigger the file;
and therefore: as many as necessary but as few as possible.