It's a combination of the larger diameter Z mount, and having the focus motors built into the lenses. Plus the new element designs sometimes require more space than the older ones. At least Nikon is now making lenses based on operational requirements instead of mainly aesthetics or convenience. Everyone used to think it was so nice that most Nikkors used the same filter size, but just think of the constraints that put on the lens designers. It was done mainly to satisfy the wants of the photojournalists who needed to change lenses often while covering a story, and didn't want to have to deal with carrying extra filters.
PF