Javascript for updating iframe height with postMessage

iframe.js

(function(){
  
  function post_height(){
    var msg = JSON.stringify({event:'size', height: document.body.scrollHeight});
    window.top.postMessage(msg, '*');
  }
  
  var onload = window.onload;
  window.onload = function(){
    if( onload && typeof onload =='function'){
      onload();
    }
    post_height();
    setInterval(post_height, 1000);
    window.top.postMessage(JSON.stringify({event:'load'}), '*');
  };
  
})(); 

top.js

(function(){
  
  function receiveMessage(e){
    var iframes = document.getElementsByTagName('iframe');
    for(var i=0; i<iframes.length; i++){
      var iframe = iframes[i];
      var src = iframe.src;
      if( src && src.indexOf( e.origin ) === 0 ){
        var data = JSON.parse(e.data);
        switch( data.event ){
          case 'size':
            iframe.style.height = (data.height+10)+'px';
            break;
          
        }
      }
    }
  }
  
  if( window.addEventListener ){
    window.addEventListener('message', receiveMessage, false);
  }
  else {
    window.attachEvent('onmessage', receiveMessage);
  }
})();

it does not shrink