1.我觉得这句应该改成198px;  

 #imageMenu ul li.big a {width:200px;}

2.而且这句可以删掉  

#imageMenu ul li a{width: 73px;}

<style>
        #imageMenu{width:500px;height:200px;overflow:hidden;}

        /*动画样式*/
        #imageMenu ul *{transition:all linear 0.2s;}

        #imageMenu ul{width:800px;height:200px;padding: 0; margin: 0; list-style: none}
        #imageMenu li{float:left;}

        #imageMenu ul li a{width: 73px;}

        /*放大后的样式*/
        #imageMenu ul li.big a {width:200px;}

        #imageMenu li a{width:98px;height:200px;display:block;text-indent:9999px;border-right:2px solid #fff;cursor:pointer;overflow:hidden;}

        #imageMenu li.landscapes a{background:url(http://img1.sycdn.imooc.com//53451c66000170b503200200.jpg) no-repeat;}
        #imageMenu li.people a{background:url(http://img1.sycdn.imooc.com//53451cc900012a0303200200.jpg) no-repeat;}
        #imageMenu li.nature a{background:url(http://img1.sycdn.imooc.com//53451d020001336503200200.jpg) no-repeat;}
        #imageMenu li.urban a{background:url(http://img1.sycdn.imooc.com//53451d370001d17203200200.jpg) no-repeat;}
        #imageMenu li.abstract a{background:url(http://img1.sycdn.imooc.com//53451d5700013fd203200200.jpg) no-repeat;}

    </style>