Animated logo header shrink on scroll

HTML

<div class="header_nav"><img src="http://placehold.it/90x90" /></div>

CSS

body {
    height:1000px;
    width:100%;
    background-color:#F0F0F0;
}

.header_nav {
    width:100%;
    height:100px;
    background-color:#666;
    position:fixed;
    top:0;
    left:0;
}
.header_nav img {
    max-height: 100%;
    max-width: 100%;
}

JavaScript

$(function(){
    $('.header_nav').data('size','big');
});

$(window).scroll(function(){
    if($(document).scrollTop() > 0)
    {
        if($('.header_nav').data('size') == 'big')
        {
            $('.header_nav').data('size','small');
            $('.header_nav').stop().animate({
                height:'40px'
            },600);
        }
    }
    else
    {
        if($('.header_nav').data('size') == 'small')
        {
            $('.header_nav').data('size','big');
            $('.header_nav').stop().animate({
                height:'100px'
            },600);
        }  
    }
});