Tag cloud font size

Hello,

I’m trying to change the font size of the tag cloud elements using the following filter hook:

function set_tag_cloud_sizes($args) {
$args = array(
	'smallest'=> 14,
	'largest'	=> 14,
	'unit'	=> 'px',
	);
return $args;
}	

add_filter('widget_tag_cloud_args','set_tag_cloud_sizes');

This code gives me the right font size, however the title of the tag cloud widget disappears.

On the other hand, if I use this syntax for defining an array instead of the previous, the title remains:

function set_tag_cloud_sizes($args) {
$args['smallest'] = 14;
$args['largest'] = 14;
$args['unit'] = px;
return $args;
}	

add_filter('widget_tag_cloud_args','set_tag_cloud_sizes');

Why do these two different ways of defining arrays give me two different results??