This is a reasonably well documented bug, introduced in Maverick (10.10), already fixed in Natty (11.04) and backported to Maverick via PPA.
The bug description has the details you need to fix this, but in summary, you can add this PPA here to apply the fix to Maverick. NB : I haven't tested this.
sudo add-apt-repository ppa:bratsche/gtk
sudo apt-get update
sudo apt-get install gtk+2.0 - 2.22.0-0ubuntu2~bratsche1
(then remove the repository, perhaps using Synaptic, or the next update you do will get all of his additional packages)
Or if you don't mind the aesthetic, you can simply edit /usr/share/themes/Ambiance/metacity-1/metacity-theme-1.xml.
Set the following values in frame_geometry_normal as desired:
<distance name="left_width" value="3"/>
<distance name="right_width" value="3"/>
<distance name="bottom_height" value="3"/>
Looks pretty ugly though!