Where are WordPress default widget files or functions located?
Those are located in the default widget class:
/wp-includes/default-widgets.phpFor more detail:http://phpdoc.wordpress.org/trunk/WordPress/Widgets/WP_Widget_Tag_Cloud.html
The widget invokes a function deeper though that generates the actual output. You'll want to take a look at the wp_tag_cloud and wp_generate_tag_cloud functions in:
/wp-includes/category-template.php
do take into account however that you'll lose your change when you upgrade wordpress (which is why I eventually did create my own shabby category-cloud widget instead of changing the default tag-cloud code).