It's not the resolutions, different resolutions is fine. It's the scaling. 1440p at 14" requires scaling because everything is too small (for me, anyway). But the portable monitor doesn't need scaling. I couldn't get it to work nicely without running two different X servers (one for each display).

I am pretty sure it's a limitation in X, the dpi setting is global, not per output. Wayland seems to have solved this, but it has other problems for me (forced vsync being the major one). So I just live in a lower resolution.

I don't really care, like some of the other posters in the thread, the lower resolution annoys me less than a 60hz refresh rate (laptop monitor is 144hz). Luckily X can handle different refresh rates for different outputs.