While viewing application screen sharing in Microsoft Skype for Business 2016 or 2019, if your display resolution is higher than the sender's, you may find the fit to window or actual size commands don't work for the shared application.
Due to technical limitations in the application screen sharing components, the client can scale down the screen sharing image, but not scale it up.
To avoid confusion when the functionality will not work as expected, the Fit to window button will be disabled when we detect that the scaling required is not possible.
To fix this issue, install the February 5, 2019, update (KB4462114) for Skype for Business 2016.
Performing desktop sharing, instead of application sharing, will leave the Fit to window button enabled and allow viewers to scale up the image.