Lightroom appears enlarged and is unusable on second screen

Hello, 

I am using Lightroom on an HP Envy Laptop, with a supporting HP desktop screen. The display setting is "extend". When I open Lightroom on the Laptop it appears OK but when I drag the window to the desktop screen for use, the app becomes enlarged with big buttons and it goes off the screen. 

It is as issue that makes it not usable. 

Can you please help with this?