This happens when the balancer is trying to move a chunk that is too big to move. It issues a "forced split" to reduce the size of the chunk. The split succeeds, but it splits the chunk on the 250000th document in the chunk. By the time mongodb tries to move the same chunk again it will be too big to move. It will then issue another "forced split". The "forced split" will result in a spilt on the 250000th document in the chunk. This pattern will sometimes go on for several hours. It will result in several hundreds or sometimes thousands of "small" chunks containing just a hand full of documents.