I'm guessing they made a mistake with their latest release. I believe the intent of the "Focus on content" feature is to embiggen the screen-share and hide the gallery completely. You can see this in the pop-up if you hover on "Focus on content." Instead of hiding the gallery, it hides participant's avatars (or webcam feeds) within the gallery rather than the entire gallery "widget" itself.

The result of this oversight is nothing changes size-wise when you select "Focus on content." It simply hides the participants. I'm guessing this was a communication failure on their developer team. Clearly the feature as it exists now is basically useless. This update also makes full-size screen sharing impossible—a real problem for many people, particularly developers who pair program and need to see very tiny things as big as possible.
My assumption is they've already realized this and are working to push it into their next release. Let's hope they see this and shake their heads in mock agreement, knowing it's a problem and they're already on top of it. Maybe.