Have you looked through your style.css file to find where the fonts are set? They might have something like 90%, 70% and so on.
Font-sizes are influenced by a parent/child relationship. For example if you have a list:
* Item One
> Sub One
> Sub Two
* Item Two
* Item Three
And your nested links feature different fonts, the percentage feature is based upon the percentage of the parent. I don't know if this is your problem, but it is really common. So if the parent Item is set for 90% of the default font, and then Sub is 80%, it is really 80% of the 90% which makes it small. If the SubSub is set for 70% (which many people do because they don't understand this), the SubSub is really 70% of the 80% Sub which is 80% of the Item which is already 90%....see where I'm going. The SubSub is tiny.
So check to see what those say and remove the %, if there is one, on the nested links you don't want to be so small and to stay the same size as their "parent".
WordPress CSS Information and Techniques will help you more.