Steps to reproduce using qtbug35647.zip:

• Click through wizard, note all buttons have large fonts except "Finish"
• Relaunch with cmd line option -s, clearing the style sheet. All buttons have large fonts.

Note this only works in 5.1.1.

Generate QWizard class/form from File->New File or Project menu.
Then in its creator try:

ui->setupUi(this);

QFont font = button(QWizard::CommitButton)->font();
font.setPointSize(50);
button(QWizard::CommitButton)->setFont(font);

This isn't going to work.

Alternatively you can try the same in promoted page's initializePage() method. Doesn't work either.
BUT changing the font size with setStyleSheet method DOES work!

Changing other QWizard buttons' font sizes also works.
It used to work for Commit and Finish buttons in Qt 5.1.1 so it seems a lot like a regression.