Eclipse Juno uses CSS to define some of its layout and the CSS class for setting the tab style is called ".MPartStack".
Here is how to find the file containing the CSS we want:
cd eclipse
grep MPartStack * -R

# edit the file that matches your operating system (e.g. for Linux it will be the gtk file)
nano plugins/org.eclipse.platform_4.2.0.v201206081400/css/e4_default_gtk.css

# find this part
.MPartStack {
       font-size: 11; <-- Change this number -->
       swt-simple: false;
       swt-mru-visible: false;