my tags showed same font size

Hi :
I have a problem regards the tag cloud I used, instead of showing different size of the font, it showed same size(actually it showed the smallest font size )

For example, in sidebar.php I wrote <?php wp_tag_cloud(‘smallest=10&largest=22’); ?>
All the tags came out with the same font size as 10, no variation

How to fix it? I want it show diff font sizes

Thanks a lot