The font size doesn't appear to be correct; why?

I'm writing a WPF app, using VS 2012. I've got a textblock on the user control, using a font family of Century Gothic and a font size of 12. It just doesn't look to me as though it is 12. So I brought up MS Word (I'm using Word 2013) and typed the same text into it, using the same font family and size, and sure enough they're not the same size. Here's what it looks like in Word:

And here's the same text from my WPF app as I'm running and testing it:

They certainly don't look the same size to me. The one from the WPF app is visually smaller. Why is that? For what it's worth, here's the XAML:

<TabItem Header="Client Demographics" FontFamily="Century Gothic" FontSize="12" />

Rod