The QIcon documentation says that if you ask for an pixmap that is -smaller- than the available sizes and no pixmap of that size has been set, then it will scale a larger pixmap down. However, it will not scale a smaller pixmap into a larger one. So if 32 x 32 is the largest size available, then you'll have to supply your own if you want something larger. If you derive from QFileIconProvider, then you can always do this scaling up yourself instead of having to invent new pixmaps and put them into resources.

the Qt sources, but I don't understand too much of the code there.
You're not the only one with that problem...