Sharding an existing small collection results in large number of chunks

Creating a small collection (about < 4M) and then sharding the collection triggers a multi-split generating a large number of chunks (possibly thousands).

Some examples:
For 20000 initial documents containing only an ObjectId and an int, inserting 7 additional similar documents will trigger a split into 1819 chunks.
For 90000 initial documents containing only an ObjectId and an int, inserting 7 additional similar documents will trigger a split into 8183 chunks.
A hard limit is reached at 8192 and with 100000 + 7 documents, no split occurs.

The behavior was observed with the following variations:

• enabling/disabling the balancer
• monotonically increasing shard key / hashed shard key
• 1 or 2 shards

The behavior did not occur on 2.6.11: only a few chunks are created (3 in the case of 20007 documents described before).