Getting "undefined" for animal size

'use strict'
var mongoose = require("mongoose");

mongoose.connect("mongodb://localhost:27017/sandbox");

var db = mongoose.connection;

db.on("error", function(err) {
    console.error("connection error:", err);
});

db.once("open", function(){
    console.log("db connection successful");
    //all db comunication goes here
    var Schema = mongoose.Schema;
    var animalSchema = new Schema({
        type:   {type: String, default: "goldfish"},
        color:  {type: String, default: "golden"},
        mass:   {type: Number, default: 0.007},
        name:   {type: String, default: "Angela"}
});

animalSchema.pre("save", function(next) {
    if(this.mass >= 100) {
        this.size = "big";
    } else if (this.mass >= 5 && this.mass < 100) {
        this.size = "medium";
    } else {
        this.size = "small";
    }
    next();
});

var Animal = mongoose.model("Animal", animalSchema);
var Elephant = new Animal({
    type: "Elephant",
    color: "gray",
    mass: 6000,
    name: "Lawrence"
});

var animal = new Animal({}); //saves default

var whale = new Animal({
    type: "whale",
    mass: 190500,
    name: "Fig"
});

var animalData = [
    {
        type: "mouse",
        color: "gray",
        mass: 0.035,
        name: "Marvin"
    },

    {
        type: "nutria",
        color: "brown",
        mass: 6.35,
        name: "Gretchen"
    },

    {
        type: "wolf",
        color: "gray",
        mass: 45,
        name: "Iris"
    },

    Elephant,
    animal,
    whale
];

    Animal.remove({}, function(err) {
        if (err) console.error(err);
        Animal.create(animalData, function(err, animals){
            if (err) console.error(err);
            Animal.find({}, function(err, animals){
                animals.forEach(function(animal){
                    console.log(animal.name + " the " + animal.color +
                    " " + animal.type + " is a " + animal.size + " animal.");
                });
                db.close(function(){
                    console.log("db connection closed");
                });     
            });                         
        });
    });
});