clarry: Do I really have to?
Well, you don't have to do anything you want to of course, but you did make a claim about me, and for that to be a fact, you need to prove it.
clarry: They asked for the games to be portable. That means once you have the files, you can run them from where ever the hell they happen to be. You could also copy the files as-is (for example, because you want to try a mod but you want to make sure it doesn't mess up your existing installation), and run that copy where ever you happened to put it.
I know what portable means, and I wasn't really addressing that aspect, other than in the context of size.
Portable like I have said, will always be much larger than installer, and a claim was made about being double, which would only be the case if the installer wasn't compressed at all, just packaged only ... which I don't believe is ever the case with GOG installers.
Anyway, the ultimate portable state is the compressed one of just a few files (EXE, BIN, ZIP, RAR). They are much better for archiving and for copying. Copying or Moving a folder of files and sub-folders takes way longer than the zipped up (packaged) version. You also don't get some of the other occasional issues you mentioned.
The only issues are extraction time and space to contain source and destination extraction content, if on the same drive. Typically, a smart person has the source file(s) on another drive, which should be the case if you are doing proper backing up.
Anyway, how do you suggest GOG provide an expanded portable version? And why should they, when clearly the majority would prefer the better archive type? To me it makes no sense to provide what is almost the same package of files.
Would you keep the downloaded package and an extracted copy as well? At some point you would have the package version and the extracted version, so you cannot avoid the space issue in the first instance ... only for maybe subsequent installs .... just a waste of time to my mind.
GOG plead they are too busy as things are, so why would they do this extra work, when it doesn't make much sense to do so?
Yes, I get it would be nice to have a package without dependencies outside, but for some games that is just not realistic, and in most cases would be down to how the developer has things set up, so not a GOG issue.
Developers seem to struggle as it is in many cases, to provide a GOG centric version of their game. You really don't want to give them another hurdle or deterrent. Getting bug fixes and updates is bad enough much of the time.