What you don’t mention is whether you are using widgets or not in your sidebar.
If yes, I’m not sure what to recommend beyond editing the core WordPress file wp-includes/widgets.php to modify how wp_tag_cloud() is called in there (in the function wp_widget_tag_cloud()). *If* you choose to do this, make a back-up copy of widgets.php in case things go awry.
If no, I don’t believe the tag cloud is originally in the default theme. But it would most likely be in the Sidebar (sidebar.php) template, if someone has added it.