The MediaWiki Tag Cloud generates a tag for every sub-category, but not article, in the given category within this wiki. The top-level category is not included in the display and the Tag Cloud does not recursively go through the sub-categories for additional categories.
The size of the individual tags are based on the number of "children", both category and articles, of each sub-category. These are normalized against the other counts and then rendered based on relative size.
Click here for all the resources.