Tags are added to assets by the publisher to help describe them. Those tags are all presented on this page as a Tag Cloud. One of the features of a Tag Cloud is that each tag varies in size based on popularity. The most popular tags are larger. Click on a tag to view all the assets with that tag.
Note: Even though you can see a Tag - you need the correct permissions to see all the assets under a Tag. Therefore you may need to login OR request access to see some assets.