When the parameter tabs are visible - namely in the picture above, Layout, Panel, Button, Color, Drag and Common - the width is dictated by those ‘tabs’ being present and drawn.
If you click on the ‘non default parameters only’ button (the concentric circles icon) - the tabs disappear and the parameter pane re-sizes, just like you would want. (though you could argue that you want the float fields to collapse even smaller than 8 decimals)
I suppose some solution that allowed the tab names to be more re-sizable would be ideal. The GLSL MAT node is murder on my screen real estate.