Why doesnt my QGridLayout resize?

[tgreaves]

This resizes perfectly ------

QGridLayout *layout = new QGridLayout(this);
layout->setSpacing(6);
layout->setMargin(11);
QPlainTextEdit* mybox = new QPlainTextEdit(this);
layout->addWidget(mybox, 0, 0, 1, 2);
setLayout(layout);

QGridLayout *layout = new QGridLayout(this);
layout->setSpacing(6);
layout->setMargin(11);
QPlainTextEdit* mybox = new QPlainTextEdit(this);
layout->addWidget(mybox, 0, 0, 1, 2);
setLayout(layout);

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This doesnt resize at all ------

QWidget* widget = new QWidget(this)
QGridLayout *layout = new QGridLayout(widget);
layout->setSpacing(6);
layout->setMargin(11);
QPlainTextEdit* mybox = new QPlainTextEdit(this);
layout->addWidget(mybox, 0, 0, 1, 2);
setLayout(layout);

QWidget* widget = new QWidget(this)
QGridLayout *layout = new QGridLayout(widget);
layout->setSpacing(6);
layout->setMargin(11);
QPlainTextEdit* mybox = new QPlainTextEdit(this);
layout->addWidget(mybox, 0, 0, 1, 2);
setLayout(layout);

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The only difference is with the 2nd example, im creating a widget* and making the layout a child of that widget which is what Qt Designer does.. How can I make this resize work using the 2nd example?