Well, they are quite small compared to a lot of other scans on Archive. Archive.org is not concerned with compressing files. Just the opposite. They are trying to preserve things. Not only that but they also take the uploaded documents and convert them into a bunch of different formats.
In general, better to have higher resolution scans than low res scans. People can compress files to make them smaller with the loss of dots-per-inch but you can't go the other way without re-scanning the document.