We have setup a 6 Shard MongoDB cluster with a replication factor of 3.
When starting the router process, default chunk size and oplog size was chosen by not specifying the values for these explicitly.

Shard3 has a chunk size of 161 MB while the rest have 60-90 MB per chunk.
All shards are similar type of instances on Amazon EC2 environment.
What we have noticed using db.<collection>.getShardDistribution() command is as follows:

Shard shard1 at shard1/<ips of shard1>
data : 38.8Gb docs : 43049426 chunks : 621
estimated data per chunk : 63.99Mb
estimated docs per chunk : 69322

Shard shard2 at shard2/<ips of shard2>
data : 40.24Gb docs : 44644092 chunks : 620
estimated data per chunk : 66.47Mb
estimated docs per chunk : 72006

Shard shard3 at shard3/<ips of shard3>
data : 102.65Gb docs : 113874252 chunks : 649
estimated data per chunk : 161.97Mb
estimated docs per chunk : 175461

Shard shard4 at shard4/<ips of shard4>
data : 54.51Gb docs : 60472368 chunks : 620
estimated data per chunk : 90.04Mb
estimated docs per chunk : 97536

Shard shard5 at shard5/<ips of shard5>
data : 50.48Gb docs : 56005174 chunks : 620
estimated data per chunk : 83.38Mb
estimated docs per chunk : 90330

Shard shard6 at shard6/<ips of shard6>
data : 46.32Gb docs : 51388397 chunks : 620
estimated data per chunk : 76.51Mb
estimated docs per chunk : 82884

Totals
data : 333.05Gb docs : 369433709 chunks : 3750
Shard shard1 contains 11.65% data, 11.65% docs in cluster, avg obj size on shard : 967b
Shard shard2 contains 12.08% data, 12.08% docs in cluster, avg obj size on shard : 967b
Shard shard3 contains 30.82% data, 30.82% docs in cluster, avg obj size on shard : 967b
Shard shard4 contains 16.36% data, 16.36% docs in cluster, avg obj size on shard : 967b
Shard shard5 contains 15.15% data, 15.15% docs in cluster, avg obj size on shard : 967b
Shard shard6 contains 13.91% data, 13.91% docs in cluster, avg obj size on shard : 967b