RoutingTableHistory currently copies the chunkMap unconditionally:

https://github.com/mongodb/mongo/blob/r4.4.0-rc12/src/mongo/s/chunk_manager.cpp#L600

And throws it away if it results in no diff:

https://github.com/mongodb/mongo/blob/r4.4.0-rc12/src/mongo/s/chunk_manager.cpp#L665

In special cases when the size of the changedChunks is 1, we can simply inspect the chunk and compare it to the chunk in the map and skip the copying all together when they are the same. This would save memory and computation in collections with millions of chunks.