The usePowerOf2Sizes documentation states that "By setting usePowerOf2Sizes, you ensure that MongoDB will allocate space for documents in sizes that are powers of 2 (e.g. 4, 8, 16, 32, 64, 128, 256, 512...8388608)"

<http://docs.mongodb.org/manual/reference/command/collMod/>

However, when there is an allocation greater than >= 4mb the allocation will be quantized to a 1mb boundary not to a power of two size boundary.

The relevant code is

<https://github.com/mongodb/mongo/blob/5a16894d45d9236acb348e0d9d805320c08c294e/src/mongo/db/namespace_details.cpp#L833-L837>