enomoto celtislab

You are using a tag cloud of the standard, and you do not want to display tags that belong to the category not related to the article. 
Try this widget. 
Based on the category of the display article, this widget gets the tag from the article categories of related range. 
For the article there is no category of post page, etc., or by setting <code>all category</code>, <code>internal link article</code>, it displays the tag by examining the related category. 
If you check the ‘internal link article’, get a category from the internal links to articles.
Select Category related range
<ul>
<li>All Category (unrelated article)</li>
<li>Current article Category</li>
<li>Include child category tree</li>
<li>Include parent category tree</li>
<li>Include internal linked article Category</li>
<li>Exclude Category</li>
</ul>
Other parameters : Display format, Sort by, Order, Font size, Number of tags to display, …
<a href="http://celtislab.net/wp_plugin_DCTagCloud/" title="Documentation in Japanese" rel="nofollow ugc">日本語の説明</a>

DynamicCategoryTagCloud

Displays the tag cloud dynamically from related articles belonging to the category of the display article

Waterloo Plugins

Tag Cloud Widget adds a simple tag cloud widget that you can use anywhere on your blog. This plugin looks at all your tags and displays them in a widget. The more times a tag is used, the larger the tag is.
Each tag is a link to the tag page (e.g. http://your-site.com/tag/your-tag). This will help with SEO.
Currently, there aren’t any options to customize the tag cloud. It should work well out of the box. In the future, there may be options to customize the widget.

Good

A tag cloud widget with links to your tag pages

DynamicCategoryTagCloud vs Tag Cloud Widget