colspec & colwidth in informaltable

Is there any special I need to do to get the colwidth attribute to colspec 
to be processed?  No matter what I set it to be, I get the same widths. 
Here's a snippet:

    <informaltable pgwide="1">
      <tgroup cols="3" colsep="1" rowsep="1">
        <colspec colnum="1" colname="c1" colwidth="2*"/>
        <colspec colnum="2" colname="c2" colwidth="1*"/>
        <colspec colnum="3" colname="c3" colwidth="5*"/>
        <thead>

I've tried it with '*' for proportional widths, inches, nothing seems to 
work.