What do you do?
Start by understanding the problem.
Your internal display has more pixels per inch than your external display this means that a say 64x64 pixel image (like an icon you might find on the launcher) will cover a lot more visable space on the 24" diagonal 1920x1080 (FHD) monitor which displays ~2 million pixels than on the 13" (QHD+) Display on your laptop which packs 5.7 million pixels into a 13" diagonal screen. This is to be expected. The higher the resolution (dots per inch) the smaller an image of a specific size in pixels will appear. The reverse is also true. The lower resolution makes the same image appear larger.
What can you do to resolve it?
There's no way to make a FHD monitor display as many pixels as a QHD+ screen. Even if you were to purchase a QHD+ External monitor with the same resolution (provided you could find one) it would likely be physically larger and still have fewer pixels per square inch than your laptop screen.
You can play with the Scale options for the displays in an attempt to normalize size at the expense of clarity, but as your laptop display has almost 3X as many pixels in ~54% of the space, I doubt you'll find it to be exceptionally effective. You might however arrive at a combination of settings that you consider suitable.