Vulkan custom resolution issue

I want to set custom resolution for my packaged game. I am using Vulkan Mobile renderer (ES3.1) becasue target platfrom is Surface 4 Pro. I am changing resolution by ResolutionSizeX and ResolutionSizeX in DefaultGameUserSettings.ini.

The problem is, when I run the packaged game, the viewport is not scaled up to the display resolution so I can see only a small game window of size I set in DefaultGameUserSettings.ini:

Is there a way I can fix this?

My DefaultGameUserSettings.ini:

[/Script/Engine.GameUserSettings]
bUseVSync=False
ResolutionSizeX=1280
ResolutionSizeY=854
LastUserConfirmedResolutionSizeX=1280
LastUserConfirmedResolutionSizeY=854
WindowPosX=-1
WindowPosY=-1
bUseDesktopResolutionForFullscreen=True
FullscreenMode=0
LastConfirmedFullscreenMode=0
Version=5