This is due to the way Windows scales the desktop depending on resolution. On Windows 7 you can only have one scaling setting for all displays, so if you have a mixture of high resolution (3200x1800) and low resolution displays (1080p) the scaling will not look right on one of the screens.
https://technet.microsoft.com/en-us/.../ff629368.aspx
Windows 8 onwards solves this by letting you have different scaling on different screens.
I assume you are using Windows 7, which is why you are seeing this issue?
Wim