Increase Height of QPushButton in PyQT

I have 2 QPushButton in the app window: btn1 needs to be 5x the height of btn2.

Problem: Tried setting the row span of self.btn1 to 5 using layout.addWidget but the height remains unchanged. did I miss out on a setting?

import sys
from PyQt4 import QtGui, QtCore

class MainWindow(QtGui.QMainWindow):
    def __init__(self):
        super(MainWindow, self).__init__()
        self.initUI()

    def initUI(self):
        layout = QtGui.QGridLayout()

        self.btn1 = QtGui.QPushButton('Hello')
        self.btn2 = QtGui.QPushButton('World')

        layout.addWidget(self.btn1, 1, 1, 5, 1)
        layout.addWidget(self.btn2, 6, 1, 1, 1)

        centralWidget = QtGui.QWidget()
        centralWidget.setLayout(layout)
        self.setCentralWidget(centralWidget)

def main():
    app = QtGui.QApplication(sys.argv)
    mainWindow = MainWindow()
    mainWindow.show()
    sys.exit(app.exec_())


if __name__ == '__main__':
    main()