Ugly fonts in qt applications which are built from source package, Ubuntu 13.04

The problem is: all applications that use Qt5 (either my own applications or Qt’s own) use very huge fonts by default. I haven’t found any Qt5’s configuration file in /home or /etc, and I don’t see any reason for it to exist, so I think it’s really a strange default value.

I’m using Kubuntu, and Qt5 is installed through official installer. The problem is found using the default theme (Fusion?). However, switching to GTK theme solves the problem.

For example, compare two attachments. One is /opt/qt/5.0.0/gcc/bin/assistant, and menu and toolbar fonts are huge, and another is /opt/qt/5.0.0/gcc/bin/assistant -style gtk, and all fonts are adequate.

The same situation exists in Qt Creator, and in my own programs that I compile using Qt5.

How to configure the font size? What can influence this?