This is probably the "inverted Electron bloat" phenomenae: Native Qt/C++ code gets 9MB in size when compiled for the web (refering to the demo http://example.qt.io/qt-webassembly/Slate/slate.html). That's even a lot of stuff to download in the AngularJS era.

However, don't be fooled: Your native C++/Qt application links to a 5MB libQtCore.so, 4MB libQtQuick.so, 11MB libQtGui.so, etc. (or .dll on Windows). But that's of course the price for GUI toolkits we pay since 20 years.

Actually it's quite surpising that they can compile a Qt application to only 9MB, comparing to the size of the desktop shared objects!