Tamaño y pelota

<html>
    <head>
        <title>TODO supply a title</title>
        <meta charset="UTF-8">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css">

        <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

        <script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.0/umd/popper.min.js"></script>

        <script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.0/js/bootstrap.min.js"></script>

        <style>
            #contenedor{
                background-color: lightblue;
                width:500px;
                height:600px;
                padding:10px;
                border:10px solid blue;
                position:relative;
            }
            .elemento{
                background-color: red;
                border: solid 1px black;
                display:inline-block;
                box-sizing: border-box;
            }
            .pelota{
                background-color: red;
                border: solid 1px black;
                position:absolute;
                top:0;
                left:0;
                box-sizing: border-box;
                border-radius: 50%;
                width:50px;
                height:50px;
            }
        </style>
    </head>
    <body style="background-color:lightgrey">
        <input type="number" id="incremento" value="10">
        <input class="btn btn-info boton" type="button" id="cambiar"  value="Cambiar">
        <input class="btn btn-info boton" type="button" id="pelota"  value="Pelota">
        <div id="contenedor">
        </div>
        <script>
            $(function () {
                var incX=10;
                var incY=10;
                $('#cambiar').click(function () {
                    var incremento = Number($('#incremento').val());
                    $('#contenedor').width($('#contenedor').width() + incremento)
                            .height($('#contenedor').height() + incremento);

                });
                $("#contenedor").mouseover(function (e) {
                    console.log(e);
                });
                $('#pelota').click(function(){
                    $("#contenedor").append("<div class='pelota'></div>");
                    setTimeout(moverPelota,100);
                })
                function moverPelota(){
                    $(".pelota").css("top","+="+incY+"px");
                    $(".pelota").css("left","+="+incX+"px");
                    
                    var top=parseInt($(".pelota").css("top"));
                    var left=parseInt($(".pelota").css("left"));
                    var ancho=$('#contenedor').width()-$(".pelota").outerWidth()/2;
                    var alto=$('#contenedor').height()-$(".pelota").outerHeight()/2;
                    if (top>alto || top<0){
                        incY=-incY;
                    } 
                    if (left>ancho || left<0){
                        incX=-incX;
                    } 
                    setTimeout(moverPelota,100);
                }
            });

        </script>
    </body>
</html>