Not sure if this is the case or not but perhaps it’s something you could look at.
In Windows, each monitor can have its own scaling setting, and if I were to guess I would say that on your 10" tablet the scaling is set to something greater than 100%. That would make controls on application windows that conform to that scaling appear larger, but controls on windows that don’t handle the scaling would appear very small.
Regardless, I would take a look at what the scaling is set to on your tablet and play with that to see if it improves the size of that ATC window. You can see what your scaling is set to on your monitors by clicking the Start button and typing “scaling” to bring up those settings.
FWIW, I run ATC and VFR windows on my second monitor all the time and the size of the text is just fine.