increasing the size of navigation bar in WPF

hi all of you,

is there some way to increase it? it's smaller if you compare with my fonts and the rest of UI stuff..

I'm talking about two buttons (left and right) which appear on the top every XAML page.

Any hint would be greatly appreciate,

Enric

Primary platform is Windows 7 Ultimate 64 bit along with VS 2012/Sql2k8 for WPF and SilverLight stuff