Digital File Sizes: Not All Pixels are Created Equal
You have a good scanner that produces files over 20 megapixels in size and a good digital camera that shoots 6 megapixel images. Which gives you the better image? Based solely on the numbers, one would be tempted to guess the scanner, but the numbers only tell part of the story.
Before delving into our opening question, suppose you also have a very cheap digital camera that shoots at only 640 x 480 (that was state of the art not that many years ago if you recall). Open a file from such a cheap camera in Photoshop and upsample it to 6 megapixels so it will be the same size as one of our supposedly "good" images. What will our upsampled file look like? I think you can guess that the file that was not upsampled will contain more detail and clearly be better looking than it. In other words, the upsampled image from the cheap camera will be carying around a lot of excess weight (fat) without added information. Relying solely on pixel count therefore doesn't work — not all pixels are the same.
Now back to our initial comparison between the good scanner and the good digital camera. The film that the scanner image was shot on is made of film grain which, if you magnify the image enough, you can actually see. Scan at high enough resolution and some of what you will be scanning will be a detailed image of what film grain looks like. Interesting perhaps, but not really what we were wanting I'd wager. Digital can also contain useless information in the form of noise, but at reasonable shutter speeds and lower ISO settings, the effect is negligible. Digitally captured images tend to be fairly clean, so most of the information they contain is real information. They have no grain.
The output of a scanner is also a second generation analog copy of the film image. Of course the result is a digital file, but it is an analog process that produces it by focusing a lens on the slide. The digital camera image, on the other hand, is an original, through and through. As anyone who's ever used a Xerox machine knows, there is at least some degree of loss in the process of making a copy.
Comparing digital camera images to scanned images can be an educational exercise indeed. The only comparison that really matters of course is the final result, but based on my experience, things are pretty darned close these days for most uses.
Don't let file size differences fool you. Look at the results instead.