See http://docs.mongodb.org/manual/reference/program/mongoimport/#cmdoption--jsonArray
it says "Used in conjunction with mongoexport --jsonArray to import data written as a single JSON array. Limited to imports of 16 MB or smaller."
perhaps it means "Used in conjunction with mongoexport --jsonArray to import data written as a single JSON array. Each document in the array is limited to a size of 16 MB or smaller."?