If the current total dataSize in the collection within a shard is smaller than given maxChunkSize, splitVector will return early:

https://github.com/mongodb/mongo/blob/r2.8.0-rc5/src/mongo/s/d_split.cpp#L357

Whereas, if there are sufficient amount of bytes in the collection, it can return with a split point as long as the average size in the chunk range is more than maxChunkSize / 2