Are Giant frames measured center to center?

[sergen]

Or Center-to-top?

 

[gclark8]

Giant frames, from the geometry page of the website, are sized from the centre of the BB to the top of the seat tube. However as they are compact frames the theoretical horizontal top tube measurement is also given on the web page.

Are you looking at road or mtb? For example, a "Small" MTB is C-T 17" and 565 mm across the top tube.