Callaway Big Bertha VFT. The Callaway Big Bertha VFT driver features Callaway's Variable Face Thickness (VFT) Technology which allows manufacture of a very thin - yet strong - clubface. One of the first drivers to benefit from the 'trampoline effect' whereby the face 'gives' slightly on contact with the ball and then, in a fraction of a second, restores itself, catapulting the ball off and increasing distance off the tee. The Big Bertha Hawk Eye VFT Titanium Driver head size is 300cc.