IPv6 is used internally a lot more than externally, so I would expect to see a LOT of commonality in the network ID fraction of the addresses- essentially all the bits of representing your IPv6 network ID get reduced to the number of bits in a compression token, in the worst case. In the moderate case, you get a few chatty machines (DNS servers and the like) where the whole address is converted to a single compression token. In the best case, you get that AND a lot of repetition in the rest of the message, and you reduce most of each message to a single compression token.
It's hard to explain if you haven't actually experimented with it, but modern variants of LZ compression are miraculous. It's like compilers- your intuition tells you hand tuned assembly is better, but compilers know crazy tricks and that intuition is almost always wrong. Same with compressors- they don't look at data the same way you do, and they work way better than your intuition thinks they would.
IP addresses are obviously part of the formatting data, not the fixed strings.