Hmm, try some tricks:
1-Right click on the background and click on "Display Settings". You will get two boxes for each monitor. Check both separately by clicking on them and see the slider for the "Change the Size of Text, Apps and Other Items" if it is on different percentage.
2-Each monitor runs its own resolution. Make sure you change it in both to the same resolution. On that same menu, at the end of the page, there is "Advanced Display Settings" in which you can do that.
I hope this helps