Project is always built 'debug' after migration qt4 -> qt5

Hi,
I want to release a new version of my scientific program. In this version, I switched from Qt 4.8 to Qt 5.6.
First, I was surprised about the large file size of the executable (14 MB), compared to 800 kB on Qt 4.8. Then I found that the CXXFLAGS in the generated Makefiles contain '-g' and, indeed, the executable can well be debugged with gdb.
However, in the project file I have 'CONFIG += release'.
Of course, I could edit all the Makefiles of the subfolders, but does anybody know how to fix the project file?
My OS is Mageia Linux 6, but it's intended for Windows and MacOs, too.

So I found the answer today. At least under Linux, always a debug build is generated, which is reasonable. But when you 'make install' your project, a 'strip' command is run on the executables, so that you have the release builds in the final folders.
In the end the Qt5 executalble is approximately 20% smaller in size than the Qt4 one.