Why are there different size options for the collections?

 As a part of setting the structure of your media library, we have collections in different sizes to help promote your content and help the end-user find what to use. At the top, you can place a large collection to promote a current campaign or a static collection for logotypes or backgrounds for easy access. Below the Large collection you can add multiple Medium collections on various topics or asset types you also want to promote. Small collections are placed below the Medium collections and don't take as much place as the other two options.

You can also create a group collection on a specific topic or category and place multiple collections to collect them at the same place.