Aurel,
On each respective platform, IUP internally uses native GUI functions, which would be WinAPI under MS Windows. When under Linux, IUP would use GTK+. Regretfully, IUP cannot yet efficiently use the modern Cocoa GUI of latest Mac OS X versions and has to utilize a Mac build of GTK+.
The beauty of multi-platform GUI libraries like IUP, wxWidgets, Qt and others is that the programmer can use the same library functions to create their visual interfaces that look absolutely native to each OS the library is used under. It's like using Ruben's window command that would create an ordinary Windows frame form, when under XP, but a beautiful dialog window, when under e.g. Mac OS X Mountain Lion or the latest Yosemite.
Regretfully, well developed multi-platform GUI libraries with lots of controls tend to be unreasonably large. IUP would be the smallest of the three, and Qt, the hugest if not entirely absurd.