QSystemTrayIcon does not scale on X11

I noticed that icons created using QSystemTrayIcon on Gnome and KDE have a fixed size in the notification area, unlike others which scale up and down with the tray. Is there any way to make the icons change size when the tray does ? The documentation says

Quote:

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On Windows, the system tray icon size is 16x16; on X11, the preferred size is 22x22. The icon will be scaled to the appropriate size as necessary.

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but it doesn't seem to happen.
I load the icon like this :

Quote:

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mywidget::mywidget()
{
...

trayIcon->setIcon(QPixmap(":/res/transparent_32.png"));
}

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I unsuccessfully tried a png, jpg and ico.

I attached a screenshot of what it looks like (the icons on the left and on the right of the big blue (scaled) music note)

Pierre