Update:

I think I nailed it, hinted by this post.

So I subclassed QProxyStyle as AndroidStyle and reimplemented QProxyStyle::pixelMetric this way:

Qt Code: 
  1. int AndroidStyle::pixelMetric(PixelMetric which,
  2.                              const QStyleOption *option,
  3.                              const QWidget *widget) const
  4. {
  5.     int metric = QProxyStyle::pixelMetric(which, option, widget);
  6.     switch (which) {
  7.     case PM_IndicatorWidth:
  8.     case PM_IndicatorHeight:
  9.         return 2*metric;
  10.     default:
  11.         return metric;
  12.     }
  13. }
To copy to clipboard, switch view to plain text mode 

and I set this style in the main function like this:
Qt Code: 
  1. QApplication a(argc, argv);
  2. a.setStyle(new AndroidStyle);
To copy to clipboard, switch view to plain text mode