Just to add my answer, it seems that the transitions need to be based on initial values and final values within the css properties to be able to manage the animation.
Those reworked css classes should provide the expected result :
.box{_x000D_
position: relative; _x000D_
top:0px;_x000D_
left:0px;_x000D_
width:0px;_x000D_
}_x000D_
_x000D_
.box:hover .hidden{_x000D_
opacity: 1;_x000D_
width: 500px;_x000D_
}_x000D_
_x000D_
.box .hidden{ _x000D_
background: yellow;_x000D_
height: 300px; _x000D_
position: absolute; _x000D_
top: 0px;_x000D_
left: 0px; _x000D_
width: 0px;_x000D_
opacity: 0; _x000D_
transition: all 1s ease;_x000D_
}
_x000D_
<div class="box">_x000D_
_x000D_
<a href="#">_x000D_
<img src="http://farm9.staticflickr.com/8207/8275533487_5ebe5826ee.jpg"></a>_x000D_
<div class="hidden"></div>_x000D_
_x000D_
</div>
_x000D_