So it is heavier but the cg is where the big issue comes in.
I'd bet that's exactly right. The Nighthawk simply felt a lot less "tippy" to me; that is, when you got the bike off balance at stand-still it didn't become an immediate weight trying to head to the ground.
By the same token, I felt the Nighthawk didn't want to change direction as quickly as the CB750 while at speed. The Nighthawk just kind of wallowed along as if in a trench or something. Which is nice on the freeway, but doesn't exactly make you eager to head to the mountains.