Menu-Font-Size

Does anyone know how to change the font size, for instance inside the menu?

I have two sites and do not understand, why the words in the menus are of a different size on each site, as I did not change anything – and that would be my next question: Is there a ways to modify the font and/or size in certain areas of the site (menu, main part etc.)?

http://www.moselromantikhaus.de (normal font size)
http://www.forstgut-lauksburg.de/ (too small font size)

Thanks in advance for your help!