Button Width Problem

HTML

<ul>
    <li><button>I'm a button</button></li>
    <li><button>I'm a button</button></li>
    <li><button>I'm a button</button></li>
    <li><button>Ooohhh....I'm a button</button></li>
</ul>

CSS

ul li {
    width:200px;
    display:inline-block;
    color:white;
    background-color:red;
    border-radius:5px;
    font-family:verdana;
    font-size:2em;
    box-shadow:0 7px 0 #600;
    padding:5px 20px;
    margin:15px;
}
ul li button {
    width:200px;
}