The bulk api checks that a document will not make a batch too large at the point of inserting the document into the batch. At the point of building and executing the command, though, any inserts without an id get this field added to them. The resulting document is never checked for size overflow, and can result in the following error.
[js_test:defragment_large_collection] Error: Converting from JavaScript to BSON failed: Object size 16793919 exceeds limit of 16793600 bytes. : [js_test:defragment_large_collection] Mongo.prototype.runCommand@src/mongo/shell/mongo.js:167:12 [js_test:defragment_large_collection] runCommandWithRetries/<@jstests/libs/override_methods/implicitly_retry_on_background_op_in_progress.js:58:19 [js_test:defragment_large_collection] assert.soon@src/mongo/shell/assert.js:366:21 [js_test:defragment_large_collection] runCommandWithRetries@jstests/libs/override_methods/implicitly_retry_on_background_op_in_progress.js:54:5 [js_test:defragment_large_collection] overrideRunCommand/Mongo.prototype.runCommand@jstests/libs/override_methods/override_helpers.js:80:20 [js_test:defragment_large_collection] runClientFunctionWithRetries@src/mongo/shell/session.js:368:27 [js_test:defragment_large_collection] runCommand@src/mongo/shell/session.js:468:25 [js_test:defragment_large_collection] DB.prototype._runCommandImpl@src/mongo/shell/db.js:167:12 [js_test:defragment_large_collection] DB.prototype.runCommand@src/mongo/shell/db.js:182:16 [js_test:defragment_large_collection] DBCollection.prototype._dbCommand@src/mongo/shell/collection.js:174:16 [js_test:defragment_large_collection] executeBatch@src/mongo/shell/bulk_api.js:904:22 [js_test:defragment_large_collection] Bulk/this.execute@src/mongo/shell/bulk_api.js:935:17 [js_test:defragment_large_collection] fillChunksToRandomSize@jstests/sharding/libs/defragmentation_util.js:75:30 [js_test:defragment_large_collection] createFragmentedCollection@jstests/sharding/libs/defragmentation_util.js:10:9 [js_test:defragment_large_collection] @jstests/sharding/defragment_large_collection.js:43:1 [js_test:defragment_large_collection] @jstests/sharding/defragment_large_collection.js:10:2 [js_test:defragment_large_collection] failed to load: jstests/sharding/defragment_large_collection.js