Also, the files may be of different format. For example, a large one might be a TIFF file with no compression, and the smaller ones might be JPEGs with considerable compression (often called "quality" as in Basic, Normal, Fine, etc).
Additionally, formats that include compression (such as essentially all JPEG, some TIFF, and some others) may compress different subjects in very, very different amounts. As only the most extreme example, if you take a picture of random noise, that will not compress very much, and it will present a relatively large file. At the other extreme, if you take a picture of a perfectly evenly lit wall with just one color, that file will compress a LOT more, possibly by a factor of 20x.
And it is entirely possible that all of the above are in play in any given scenario.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!