The column width is required when using the 'fitColumns' in datagrid. Please try this code instead.
<script type="text/javascript">
$(document).ready(function(){
$('#dg').datagrid({
columns:[[
{field:'id',title:'id field',width:100},
{field:'code',title:'code field',width:100}
]],
fitColumns: true
});
});
</script>