Here is a screenshot of the Xfce appearence settings in 18.04 (left) compared to 16.04 (right):
As you can see, the same font, font size, hinting, and DPI is selected.
In the bottom right is an overlay of the two showing that the word "Style" renders approximately 1 pixel taller and wider on 18.04. Since it is only 10 pixels tall to begin with this represents a 10% increase in size.
In addition, some other fonts render smaller in 18.04 than they do in 16.04 (not pictured, but they do.)
Why does this happen?