Display All Tags
The number of Tag Cloud widget tags are limited to 45 by default. Default format of the cloud display is flat. To set number to unlimited and format to list:
function set_tag_cloud_args($args) {
$args = array('number' => 0, 'format' => 'list');
return $args;
}
add_filter('widget_tag_cloud_args','set_tag_cloud_args');