Mongodb chunksize issue
As per MongoDB documentation Modify Chunk Size in a Sharded Cluster The default chunk size for a sharded cluster is 64 megabytes.

As MongoDB documentation here New in version 2.6: MongoDB provides the mergeChunks command to combine contiguous chunk ranges into a single chunk. See Merge Chunks in a Sharded Cluster for more information.

Note: Be careful when splitting data in a sharded collection to create new chunks. When you shard a collection that has existing data, MongoDB automatically creates chunks to evenly distribute the collection. To split data effectively in a sharded cluster you must consider the number of documents in a chunk and the average document size to create a uniform chunk size. When chunks have irregular sizes, shards may have an equal number of chunks but have very different data sizes. Avoid creating splits that lead to a collection with differently sized chunks.

For further your ref here