This is actually an optical illusion. I learned about this fun little phenomenon in a typography class in art school way back in 1993-94. You can actually test this theory out yourself – set x point type in Illustrator black text on white and then another (same size) reversed out. Now print it. And stare at it. The white text on black is going to look fatter. Its just the way our eyes perceive it.

I do like this tip though.