Two monitors with same resolution but very different DPI

I am using Manjaro on a Lenovo P51. The internal monitor is 1080p and so is the external. However, the internal is 144 dpi and the external 70 dpi. I run KDE and global scaling is set to 150% which makes the icons, etc. a pleasing size for me. The NVIDIA GPU uses nouveau

➜  ~ lspci -k | grep -B 2 nouveau
01:00.0 3D controller: NVIDIA Corporation GM107GLM [Quadro M1200 Mobile] (rev a2)
        Subsystem: Lenovo Device 224d
        Kernel driver in use: nouveau

and the Intel GPU uses i915

➜  ~ lspci -k | grep -B 2 i915 
00:02.0 VGA compatible controller: Intel Corporation HD Graphics 630 (rev 04)
        Subsystem: Lenovo Device 224d
        Kernel driver in use: i915

The Optimus setup is this:

➜  ~ xrandr --listproviders 
Providers: number : 2
Provider 0: id: 0x43 cap: 0xf, Source Output, Sink Output, Source Offload, Sink Offload crtcs: 3 outputs: 1 associated providers: 1 name:modesetting
Provider 1: id: 0x80f cap: 0xf, Source Output, Sink Output, Source Offload, Sink Offload crtcs: 4 outputs: 3 associated providers: 1 name:modesetting

I have looked at a great number of answers and recommendations, but when I include --scale 2x2 the frame buffer grows, but the external monitor only displays the upper left quarter. How can one get the scaling on the external monitor in this case?