Every once in a while though, data chunks go out of balance. The Mongo team is working on making the auto-rebalancing a little more intelligent in future releases. In the meantime, you can manually move chunks around between shards to keep things in balance if needed.
As per the article the trick is accomplished using:
moveChunk : [namespace],
find : [data spec],
from : [source shard],
to : [destination shard]
Original title and link: MongoDB Tip: Manual Shard Balancing (NoSQL databases © myNoSQL)