[Memorado]

Why is this happening? Why 800x600?

you need to use advanceguisettings.xml and put a another value to height and width (go check wiki).   

- Search on the forum did not shown any results on "advanceguisettings".
I found only topics on "advancedsettings".
Unfortunately, there is very little information on the Kodi interface resolution settings for this file.

I tried this advancedsettings:

<advancedsettings>
    <loglevel> 2 </ loglevel>
    <fullscreen> false </ fullscreen>
     <window>
      <width> 3840 </ width>
      <height> 2160 </ height>
    </ window>
</ advancedsettings>

But this absolutely did not affect anything, and did not fix the "800x600 HDR" problem.
Perhaps it just does not work.