[{"sweatersize":"Medium"},{"sweatersize":"Medium"},{"sweatersize":"Medium"},{"sweatersize":"Large"},{"sweatersize":"Large"},{"sweatersize":"Small"}]

db.collection.aggregate([{"$group":{"_id":"$sweatersize","count":{"$sum":1},"sweatersize":{"$first":"$sweatersize"}}},{"$unset":["_id"]}])