sounds like your gpu is doing some hdmi scaling

In Q1 that stuff is done independend of the actual video resolution. IIRC it is set by
vid_conwidth
vid_conheight

those values must be set having the same ratio as your video resolution.

The variable names may have changed in newer versions of ezquake or maybe can be set by other means however those should work nevertheless.

Example
vid_conwidth 800
vid_conheight 600
gives 4:3, same with
vid_conwidth 400
vid_conheight 300
etc, you get the idea.

I really dont. Which values for 16:9 then?

Try this, is for the Qworld client but should be the same?

https://www.quakeworld.nu/wiki/Widescreen_Guide