Adobe announced a new set of fonts called Source Han Sans ( http://sourceforge.net/adobe/source-han-sans/wiki/Home/ ). Weights of Full-width CJK punctuations should be same as a single character but Qt 4.8.6 and Qt 5.3.1 (downloaded from qt-project.org) show them in half-width with this font family.
Here is a picture that explains the issue quite well. It shows Qt 4.8.6 and official Qt 5.3.1 (downloaded from qt-project.org) wrongly rendered it but Qt 5.3.1 from Archlinux repository can do it right.
According to Dr. Ken Lunde from Adobe, this may because Qt invoked 'kern' GPOS feature automatically. However, 'pwid' feature should also be invoked with 'kern' but actually not. Then punctuations fallback to half width via 'palt' GPOS feature. Fonts without 'kern' and 'palt' don't affected by this issue.
Please see here for more information: https://github.com/adobe-fonts/source-han-sans/issues/37
Code for repeating this bug:
Thanks for tracking this bug.