The main reason that internal drives keep increasing in size is not because OS X has got larger (it has, but not by that much), nor that modern apps and document formats need more space (they do, but not that much). No, the main reason is that each time that we replace an old Mac with a new one, we have to migrate all the dross and crud from the past.
Take this iMac, which I bought in December 2015. I have more than a few files on here which were created in the early 1990s, and should already have had their twenty-fifth birthday parties. I dread to think how many documents have not been opened for more than ten years.
If I were better organised, I would of course have archived them away on archival-quality optical disks, tucked those in cool, dry and dark storage, and probably forgotten where they were and what was on them. But would it have occurred to me to ensure that they were saved in archival file formats? Probably not.
Many documents which were saved from ClarisWorks/AppleWorks are already very hard to open. Try the current versions of Pages or Numbers, and you will either not even be offered the chance, or the modern app will burp back blank refusal. If you do ever need to open an old proprietary document format, your first port of call should be LibreOffice: I have never seen an app which claims to be able to open so many different file formats, and yes, they do include old ‘works’ formats from both Apple and Microsoft.
Matters get even worse when the files were saved in the proprietary formats used by long-extinct apps like Aldus Freehand or Adobe PageMaker. Classic Mac OS formats like PICT are also fast becoming extinct, although at least Preview coupled with QuickTime seems happy with them for the present.
Software vendors are usually good at backward file format compatibility, but there comes a point where keeping all the code to support that is just not efficient. Most drop old format support silently, hoping that no one will notice. The few who will, can only kick themselves for not using a more enduring format, when they do try to access such old archives.
Like so many things in life, whether you are consciously saving documents away to an archive, or just leaving them in your Documents folder, you must think ahead. How will you open or import them in ten, twenty, or even fifty years time? If you cannot, then they will not have justified their ground-rent on successive hard drives over those years, and you may as well have trashed them and re-used the disk space.
The solution is to spend a little while each day working through the folders where you keep your old dross and crud, opening the files up, and saving the contents to common, enduring formats such as text, CSV, RTF (be careful here, it is not as standard as you might think), JPEG, PDF, and the like.
When you are going to archive old documents, do the same checks on all the files which you are preparing to archive. By all means keep a copy of the originals, but ensure that you can always access their content. Otherwise there is little point in archiving in the hope that someone will step forward with a magical conversion tool when you do want to open them, or a bureau will save the day.
If you cannot retrieve, there is no point in archiving in the first place.