Right. The reference IPv6 packet header is only twice as long as that for IPv4 (320 bits vs 160). What is crucial, combined with the fact darkr points out that

Right. The reference IPv6 packet header is only twice as long as that for IPv4 (320 bits vs 160). What is crucial, combined with the fact darkr points out that since they include the link layer the IPv6 protocol deals with more of the network architecture, is that IPv6 headers are much simpler, with only 8 fields, the 6 non-address fields all in the first 64 bits, while IPv4 has 14 fields, with the 12 non-address fields taking 96 bits, and they are better designed with attention to what routers find most important. This simplicity offers routing subsystems increased flexibility in the way they can be put together and handle their workloads.