I run dual screen on my laptop.  If memory serves; 3 things may have an impact.  First set your new monitor to 1600 x 1200 and see if it clears up, I seem to remember something about the ratio having an affect on resolution 1.33:1 vs 1.5:1 respectively for your monitors.  If this is the case then the resolutions might need/want to match.  The other is in the monitor menu itself. I don't recall what but I do recall some monitors have settings which affect resolution.  Finally, it may just be the quality of the monitors, something beyond your control.  I have seen both, good picture quality monitor designed for graphic work and color matching and a $100 special used more for just cli use in administration trying to be the former.  This is affected from the pixel-per-inch count from the manufacturer vs. dots-per-inch resolution in software for which there is a difference, though usually invisible to us.  Gnome has a tweak tool for high resolution, gnome-tweak-tool from yum.  This article may help: https://www.pcworld.com/article/2911509/how-to-make-linuxs-desktop-look-good-on-high-resolution-displays.html 

-- Fred