I hope you don't mind, but I looked through the scripts and the issue was the configuration variable in the gui.rpy file. Specifically, the "config.adjust_view_size" variable.
It seems to prevent the game resolution from scaling with the window size when switching to fullscreen, at least on my 1920x1080 monitor.
A quick and dirty fix would be to comment out the variable, but I'm not sure if that somehow affects the rest of the game's GUI.
For now, if anyone wants to apply the fix without editing the files themselves, you can download it from my Google Drive here: https://drive.google.com/drive/folders/1TscDK_T4BJ-6LTdAQEL7SY83wFzg1Xcr?usp=sharing.
You can then overwrite the scripts.rpa file in the game folder (e.g."days-of-saturn-win-linux\daysofsaturn-1.0-pc\game\scripts.rpa" for PC). I hope it helps!