Example 29

If you want the Scaffold’s child to be exactly the same size
as the Scaffold itself, you can wrap its child with
SizedBox.expand.

Note:
  When a widget tells its child that it must be of
  a certain size, we say the widget supplies tight
  constraints to its child.

Tight vs. loose constraints

It’s very common to hear that some constraint is
“tight” or “loose”, so it’s worth knowing what that means.

A tight constraint offers a single possibility,
an exact size. In other words, a tight constraint
has its maximum width equal to its minimum width;
and has its maximum height equal to its minimum height.

If you go to Flutter’s box.dart file and search for
the BoxConstraints constructors, you’ll find the
following:

BoxConstraints

tight

Size

size