Sorry, stupid me. Of course when I enlarge the window it changes. But I don't like having fullscreen browser windows. Most of the time it's of no use anyway because lines get far too wide for the text being readable comfortably. And in GTDNext a large browser window creates a lot of empty space in the middle.
But because the icons in the toolbar are quite large and have a lot of free space in between, I have to live with either fullscreen browser windows or multiple lines in the toolbar.