The Center tells the Container that it can be any
size it wants, but not bigger than the screen.
Since the Container has no child and no fixed size,
it decides it wants to be as big as possible,
so it fills the whole screen.

But why does the Container decide that?
Simply because that’s a design decision by those who
created the Container widget. It could have been
created differently, and you have to read the
Container documentation to understand how it
behaves, depending on the circumstances.

Example 7

The screen forces the Center to be exactly the same
size as the screen, so the Center fills the screen.

The Center tells the red Container that it can be any size
it wants, but not bigger than the screen. Since the red
Container has no size but has a child,
it decides it wants to be the same size as its child.

The red Container tells its child that it can be any size
it wants, but not bigger than the screen.