I am attempting to save documents to a mongoDB cluster (sharded replica sets) and am having a strange issue. I am using pymongo 2.7.2 and TokuMX 1.5 mongodb 2.4.10.

When I attempt to save (overwrite) existing documents I am getting an exception that looks like the document I am saving is too large:

```
doc = db.collection.find_one()
db.collection.save(doc)

pymongo.errors.OperationFailure: BSONObj size: 18798961 (0x71D91E01) is invalid. Size must be between 0 and 16793600(16MB) First element: op: "u"

```

However this works fine:

```
doc = db.collection.find_one()
db.collection.remove({'_id': doc['_id']})
db.collection.save(doc)

```

The document in question is about 9mb, so it looks like when I attempt to replace the document it is somehow doubling the size of the document, exceeding the 16mb limit.

Any ideas as to what could cause this behavior?