How can I download macOS High Sierra's full installation file?
For what it's worth: I have encountered a similar issue: a very small download file (14,5 MB), that launched automatically after a few seconds... tried it twice (both times the installation failed giving me some error about corrupted information on the server - I'm not sure what it meant but it might have been related to the fact that I was logged onto the MAS with a different account than the one I signed up for the public beta with). Anyway I just retried again and just on a whim before clicking download I pressed alt on the keyboard... now it's downloading the full thing.
This all happened in the space of 10 minutes.