According to this driver spec "[m]ore than 16MB worth of inserts are split into multiple messages, and error indexes are rewritten." This is in addition to splitting bulk ops that contain > 1000 operations into multiple ops.

The mongo shell does split bulk ops that contain > 1000 operations into 1000-op batches. But when the bulk op exceeds 16MB, it issues an error:

2016-03-14T11:09:57.077+0200 E QUERY    [thread1] Error: Converting from JavaScript to BSON failed: Object size 16795903 exceeds limit of 16793600 bytes. :
DBQuery.prototype._exec@src/mongo/shell/query.js:112:28
DBQuery.prototype.next@src/mongo/shell/query.js:283:5
Bulk/executeBatch@src/mongo/shell/bulk_api.js:853:16
Bulk/this.execute@src/mongo/shell/bulk_api.js:1139:11
@bulk.js:31:1

The python and C drivers, on the other hand, work as expected. They split bulk ops that either contain > 1000 ops or are > 16 MB into smaller batches.