Can I have toolbar button with different sizes?

[Maurizio Colucci <seguso forever tin it>]

I did some experiments with gtk toolbars (actually in gtkmm :-) ) and I found 
that all toolbar buttons always have the size of the largest button. 

But Nautilus and evolution seem to have a toolbar with unevenly sized buttons.  
How can I obtain such an effect?

Thank you!

(See my free file manager http://segusoland.sourceforge.net)

maurizio