If you saved all your chips scans in the bit-map format as they come from the scanner, the resulting files would all be the same size, Mike. However, most people use one of the image-compression schemes to reduce the amount of data stored for a scan. The most common are JPEG (.jpg) and GIF (.gif). These compressed files are produced using an aglorithm that looks for areas of your pic that are the same and replacing the long strings of identical information with a shorter code that represents the information in this area. You often get files that are compressed by 10x or more compared to the original (.bmp) file. Your browser has logic to decompress the file when you view it and display an image that is (almost) identical to the original scan.

Images that contain large areas with no detail, or with limited colors will compress the most.
Also, you can specify in most graphics utility programs how much to compress using a 'quality factor' that usually ranges from 0 to 9. Quality 0 gives the most compression, and is usually fine for displaying chips on a PC monitor. If you should happen to be scanning a piece of text (like a newspaper clipping) you would probably have to use a higher quality factor to read the type.