When drag from 1080p to 4K monitor, most windows double in size

[Edward Porter]

I have three monitors running on a powerful Nvidia video card, one is 1080p, one is 4K (3840 x 2160), and one is 1680 x 1050. In windows 7 I extended the desktop across them and when I dragged windows between them they stayed the same resolution and approximate size in each window. I just installed windows 10 yesterday. When I drag an app window into the 4K screen, once it gets halfway into that screen, it suddenly approximately doubles in size. For most apps all the text and graphical elements double in size in the x and y direction, at becomes obviously pixelated. But on web pages, the web page text expands in size, but does not normally become pixelated (although in some web pages even the web page text does not expand.

This greatly decreases the usefulness of my 4K screen, because it greatly decreases the number of windows and the amount of information it can display at one time. Does anybody have any idea how to fix it, or do I have to go back to window 7.