Fill Screen not filling screen on second monitor (MAC)

[ultrachrome]

It's been a long time since working on Photoshop on a Mac so forgive my ignorance please. 

I'm just setting up an additional workstation on a 2017 Macbook pro we had laying around and I'm keeping the tools on the Macbook screen and the image on a second monitor but when working in the tabs view mode, the green "fill screen" button isn't working and in order to fill the screen I need to grab the the corner and drag every photo to fill the screen and once I resize or crop, it again no longer fills the screen and that tab bounce back to only the size of the picture with the window no longer filling the screen. It is working properly if I have the picture on the Macbook monitor but not on the second monitor. Other programs the fill screen is working on the second monitor. 

What am I missing. This works fine on my PC so I suspect I'm just missing a setting somewhere. 

Does anyone know wh

[S_Gans]

I'm thinking this may not be a Photoshop setting, but rather, a Mac setting. I just tried what you said, using my iMac to a 2nd monitor, and it worked fine. What OS are you using on the 2017 Macbook pro? Is there a big discrepancy in the screen resolution between the MBP and the 2nd monitor?

[ultrachrome]

Thank you S Gans

The OS is Big Sur 11.5.3 and it turns out that this is a 2014 model not the 2017 I thought it was so this is the newest OS possible I think. 

Yes, there is s difference in screen resolutions with the second larger screen being lower than the labtop screen at 1920x1200.