It's 100% not a cable issue. I just today had openelec running just fine. With the modified xorg.conf file. Clean install of LibreElec and I'm back to a quarter screen. The previous link was someone else who had a similar problem.
Yes, this is no cable issue for sure. The expected screen modes are in your xorg.conf file, and the screen modes you actually get are in your guisettings.xml file (whitelist part). So, there is a difference. Can you post both files, please? Which resolution / vertical frequency do you use when you get a too small display?