The max window size for zlib is 32 KB, so I don't think the default sharding at 128 KB would change much. You can pass the -b parameter if you find out a bigger shard works better on your data.
If you are looking for details of the design of pigz, there is a very well-documented overview in the source of pigz.c:
If you are looking for details of the design of pigz, there is a very well-documented overview in the source of pigz.c:
https://github.com/madler/pigz/blob/master/pigz.c#L187