It's pretty important when asking why something doesn't work, that you tell us what versions of everything you're using. What version of Office, which operating system, etc.
IF you're using the equation editor also known as "Microsoft Equation 3.0" (this is the one you access by clicking Insert > Object, or in Word 2008 by clicking the Equation Editor icon on the toolbar), click on Format > Spacing, and adjust the item labeled "Fraction bar thickness".
Bob Mathews
Design Science