[{other_property:"foo",image_list:[{id:"fooid",name:"fooname",bucket:"foobucket",path:"foopath",size:"322300"},],other_property2:"foo2"}]

db.collection.update({image_list:{$exists:true},"image_list.size":{$type:"string"}},[{$set:{image_list:{$map:{input:"$image_list",in:{$mergeObjects:["$$this",{size:{$toInt:"$$this.size"}}]}}}}},{$unset:"image_list.id"}])