I'm testing the MongoDB Sharding with a Sharded Collection and a forloop to insert arround some documents to see how splitting and moving works.

After 10K documents MongoDB has split my collection in 3 chunks, well before 64Mb (the default chunkSize):

mongos> db.users.getShardDistribution()

Shard rs0 at rs0/mongod00.local.net:2100,mongod01.local.net:2101
data : 1.06MiB docs : 10000 chunks : 3
estimated data per chunk : 364KiB
estimated docs per chunk : 3333

Totals
data : 1.06MiB docs : 10000 chunks : 3
Shard rs0 contains 100% data, 100% docs in cluster, avg obj size on shard : 112B

It's not a problem but with this, I don't understant the interest of the chunkSize parameter.