Its something to do with dx12 and fullscreen scaling and having the games resolution different to your PC resolution.

Look in your nvidia settings and turn on screen scaling.

another possibility is the game cannot read your .ini files in folder
Documents\My Games\Fallout4
and is using the default settings in folder
...\Steam\steamapps\common\fallout4\fallout4\fallout4prefs.ini
iSize H=720
iSize W=1280
bFull Screen=0

Get rid of any leftovers from mods. Uninstall the game, clear the entire folder, and then reinstall the game. This includes removing the old .ini files and anything in AppData\Fallout4\, too. If you previously had mods installed and now brushed the update over it will it cause all sorts of problems, including what you are seeing now.

Another reason can be that you have High DPI Scaling enabled. Set it back to 100% or change the properties of Fallout4Launcher.exe and Fallout4.exe to override the High DPI Settings (right-click the files, see Properties->Compatibility->High DPI Override).

Read here for other issues:
https://steamcommunity.com/app/377160/discussions/0/4364626348100218422/#c4364626348100326950

The "zoomed in" issue is caused by Windows Display Scaling enlarging the game window. You need to override the high DPI settings for the game executable and run it in a maximized borderless window.

Step 1: Fix Windows DPI Scaling
Open Steam, right-click Fallout 4 > Manage > Browse local files.

Right-click Fallout4.exe and select Properties.

Go to the Compatibility tab and click Change high DPI settings.

Check the box for Override high DPI scaling behavior.

Ensure the drop-down menu is set to Application.

Click OK, then Apply.
(Repeat this for Fallout4Launcher.exe just to be safe).

This fixed it for me