[{"username":"joy","size_info":[{"size":"M","width":100,"height":102},{"size":"M","width":102,"height":104},{"size":"S","width":80,"height":82}]}]

db.collection.aggregate([{$unwind:"$size_info"},{$group:{_id:{username:"$username",size:"$size_info.size"},actual_size:{$push:{width:"$size_info.width",height:"$size_info.height"}}}},{$group:{_id:"$_id.username",size_info:{$push:{size:"$_id.size",actual_size:"$actual_size"}}}},{$project:{_id:0,username:"$_id",size_info:1}}])