My animation properties are not working simultaneously
<p>I am trying a simple animation of div which animates when page loads. Below you can see the GIF of the animation.</p>

<p><img src="https://i.stack.imgur.com/lnThV.gif" alt="enter image description here"></p>

<p>Here is the code </p>

<p><div class="snippet" data-lang="js" data-hide="false" data-console="false" data-babel="false">
<div class="snippet-code">
<pre class="snippet-code-css lang-css prettyprint-override"><code>@keyframes newActivity{      
    to{        
      position: fixed;
      z-index: 100;        
        top: 18%;
        left: 10%;        
        width:70%;
        height:80%;    
   }

}
.div:first-child {    
    animation-name:newActivity;
    animation-duration:5s;
}

div {      
    width: 290px;
    height:200px;
    float:left;
    margin-right: 10px;
    margin-bottom: 10px;
    box-shadow: 0 1px 4px rgba(0, 0, 0, 0.27), 0 0 5px rgba(0, 0, 0, 0.06);
    -webkit-box-shadow: 0 1px 4px rgba(0, 0, 0, 0.27), 0 0 5px rgba(0, 0, 0, 0.1);
    -moz-box-shadow: 0 1px 4px rgba(0, 0, 0, 0.27), 0 0 5px rgba(0, 0, 0, 0.06);
    background-color:red;
    border: 1px solid #BCBCBC;
    overflow:hidden;
    z-index:-10;  
}</code></pre>
<pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;div class='div'&gt;&lt;/div&gt;</code></pre>
</div>
</div>
</p>

<p>You can see that first it increases the width and adjust position and increase the height(Without animating it). Why is this happening? I want to animate all of these properties simultaneously. </p>