Crispy78: Afraid I don't know enough about what is available on Macs though, to allow for a multi-part installer that the average user can run easily without having to get into the terminal / command-line. Maybe that is awkward on Macs?
It is awkward. Unlike Windows, where
every file is executable simply depending on the filename (the last dot and letters/digits thereafter, specifically), neither NextStep (which formed the basis of Mac OS X) nor Unix-likes (a group of which OS X is part) behave in the same way (neither did the original System/Mac OS, but that's a fourth way not relevant now). Actually, NextStep behaves similarly (to be expanded upon), but still too different to be useful in the case of transerred files.
Inherited from Unix-likes: A file is not executable by having a specific filename, but by the user having permission to run the file as executable, this metadata is not part of the file itself, and as such is not transferred over the network but has to be set locally.
Inherited from NextStep: What the user sees as an "application" is
usually (I have seen exceptions, but do not know if this is still possible) not a single file as in Windows (where it's indicated by the
filename extension ".exe"), instead it's a folder with a host of files. The
folder has the extension ".app", which prompts the OS to treat it specially (other similar cases exist, ".pkg" is one, usually used for installers), making it slightly more difficult to enter the folder to get at the files within (unless you're using the terminal, which is part of the underlying Unix, not the NextStep-based graphical interface).
Do note that in both these cases (".app" and ".pkg") I'm talking about folders, not files named such - and as we (most of us, at least) know, folders are not transferable over the net, only files with content (actually, just the contents, and optionally other data which doesn't have to be honored such as timestamp and suggested filename, not the actual file). While there (obviously) are ways to build multipart installers for OS X, the actuall installer itself cannot be transported as-is, because it is an application (a folder, named .app or .pkg), it needs to be merged into a single file somehow - a zip file works, but the most common way by far is a disk image, specifically the .dmg format (.iso works just as well, actually). This file would need to be extracted before installation can begin and the user would have to put the installer "executable" and all the installer data files in the same folder (so they can find each other) or use several disk images and handle it as a multi-disc installer - both these are likely (sadly, in my opinion) more advanced file managing tasks than most users are willing to handle, they want to just double-click
a single file (ie, not 10 of them to do one thing) and have the expected result happen, or be given a guide on what to do next.
Instead, GOG
don't actually use installers on OS X, but a single disk image (large files
can be split up and merged, but that'd require third-party software the user is unlikely to have, and - again - introduces advanced file managing) which opens on a simple double-click, with the whole application (game) in a single .app folder inside, to be dragged-and-dropped to the Applications folder (or elswhere, but that's where most users learn to put their applications).
Crispy78: Afraid I don't know enough about what is available on Macs though, to allow for a multi-part installer that the average user can run easily without having to get into the terminal / command-line. Maybe that is awkward on Macs?
JMich: I'd suggest taking a look at GOG games with DLCs that work on Mac, since the DLCs are extra files. You could (in theory) do the same with regular games, requiring the data from parts 2 to # to be added to part 1, but you'd still end up with one huge file on your computer.
In theory, yes, but that would require opening all the disc images (as if you had half a dozen or more optical disc readers) and run the installer (because simple drag-dropping the game .app isn't an option when split over several files, you need an installer) from the correct one when all are mounted. Quite common practice back when floppy discs were still used, especially for pirated software (though Stuffit ".sit" archives were more common for legal use, I'm quite sure - just about everyone had Stuffit (the WinZip of the Macintosh at the time), but far from everyone had a disc imaging application). Floppy disc images then were as the iso's and dmg's of today.