You're right, it isn't just determined by free space. The download got stuck on the MacBook Pro and I had to cancel, trying again resulted in only the 7.8 MB installer app.
When I tried yet again AFTER I had successfully installed Sierra, then I was finally able to download the full one.
As someone who keeps offline backups of every Mac App Store major OS X version (easy to install whichever one I need), I definitely find this change to be a bit of an annoyance. Would be nice to be guaranteed the full installer instead of the bare one.
Also wonder if anyone has tried createinstallmedia with the 7.8 MB installer app. Does it download the OS portion or simply fail to work?
In the meantime I tried it 50 times, I always got the 5 MB download :-/!!!