fill

The GridBagConstraints fill field determines whether and how a component is resized if the component's display area is larger than the component itself. The mnemonic constants you use to set this variable are

GridBagConstraints.NONE

Don't resize the component

GridBagConstraints.HORIZONTAL

Make the component wide enough to fill the display area, but don't change its height.

GridBagConstraints.VERTICAL

Make the component tall enough to fill its display area, but don't change its width.

GridBagConstraints.BOTH

Resize the component enough to completely fill its display area both vertically and horizontally.

For the display area of the calculator screen use

displayConstraints.fill = GridBagConstraints.HORIZONTAL;

because you'd like the screen to show as many digits as it can.