Apple has provided the UICollectionViewFlowLayout class for us developers, which should be enough to solve the 'typical use case' of collection views. However, I believe you're correct in your assessment that the default layout does not allow you to create this tag cloud effect. If you need something different from the normal flow layout, you'll have to write your own subclass of UICollectionViewLayout.
Apple covers this topic in their 2012 WWDC session titled, "Advanced Collection Views and Building Custom Layouts"
Some additional Apple docs: https://developer.apple.com/library/ios/documentation/WindowsViews/Conceptual/CollectionViewPGforIOS/CreatingCustomLayouts/CreatingCustomLayouts.html
At the risk of seeming biased, I also wrote a quick blog post running through the basic steps: http://bradbambara.wordpress.com/2014/05/24/getting-started-with-custom-uicollectionview-layouts/
Hope that helps.
