Q: auto resize 5KiMac to 20" cinema screen

I use to have an MBP running a 20" cinema screen.

I just bought a 5KiMac and using the 20" cinema screen as 2nd screen.

It works fine.

but, when I drag and drop a open app over to the small screen, it does not automatically resize the page to fit the smaller screen.

It's kind of stupid and frustrating.

Is there a way to do that?

Thank you all in advance.