That's good to know, although there is a section on the forum just for letting people know what devices work with a Surface maybe this post will get moved there... I really want to see Microsoft bringing out their own beautiful display we could extend to, but I don't think that's likely.
No. It is because Windows applies different DPI on each display, and scales like a picture, the content of the program that is on a display not matching the DPI setting of the primary display.
To fix this problem, set both displays to the same DPI level, or better yet, 100% DPI, which will also make non-high-DPI aware software look sharp as well.
Right-click on the desktop, and select "Screen resolution", then on "Make text and other items larger or smaller", and slide the bar to right most (smaller), which is 100% DPI.
Hit Apply and restart your system.