Yes, ZX14 is bigger diameter.
Measured closest to the exhaust port, the 06/07 header measures 1.400" ID / 1.500" OD. Looks like the 08 ZX14 is the same (harder to tell, as it's on the bike).
The 2010 C14 header measures 1.375" ID / 1.475" OD, but it was more difficult to measure accurately (not as even surfaces as the ZX). Most spots I measured were smaller diameter.
Hope this helps.