The release notes mention that MongoDB 2.6 has usePowerOf2Sizes enabled by default and how to turn it off at the instance level. However it is not clear what effect this has in practice. Therefore please include this excerpt which is mentioned elsewhere in the newCollectionsUsePowerOf2Sizes parameter notes :

New collections include those: created during initial sync, as well as those created by the mongorestore and mongoimport tools, by running mongod with the --repair option, as well as the restoreDatabase command.