Distro: Linux Mint 19.1 Tessa (64 bit)
GPU: Vega 56
Driver/LLVM version: Mesa 19.1.0-devel/9.0.0
Kernel version: 5.1.1-050101-lowlatency
Proton version: 4.2-3
I could replicate the problem with a 2560x1600 resolution monitor.
steam-561600__proton-3.16-4.log
steam-561600_proton-4.2-3.log
system_info.txt

Update: Actually I just tested and this happens regardless of your monitor resolution. Even if you have a 1080p monitor and select a lower resolution this problem occurs.
No changes with Proton 4.2-7
I have this happening in Doom Eternal when using an external monitor.
proton 4.2-7x1 2019-06
Compatibility Report
System Information
GPU: RX Vega 64
Driver/LLVM version: mesa 19.0.4/8.0.0
Kernel version: 5.1.1
Link to full system information report as Gist: https://gist.github.com/amezin/27122ea029f8fa0b6b979d5ca93c9c1e
Proton version: 4.2-3
I confirm:
steam-561600.log
Symptoms
In fullscreen mode the game occupies only a quarter of the screen, when game resolution is set to 1920x1080 and screen resolution is 3840x2160
Reproduction
This game has a native port, but it is almost unplayable because of graphical glitches, so I run the game using Proton.