The problem is actually your themes h3 setting.
You can adjust this by adding a font-size: attribute to the #tooltip h3, #tooltip div style (around line 19 in lightbox-gallery.css)
was:
#tooltip h3, #tooltip div { margin: 0; }
is now:
#tooltip h3, #tooltip div { margin: 0; font-size: 10px; }
set the 10px to whatever size floats your boat ??
Cheers!
