iOS UICollectionView原型单元格大小属性被忽略

I'm using a UICollectionView with two prototype cells. The prototype cells have different widths and contain different controls (image view and web view). I'm definitely returning the correct prototype cell for a given index (all the cells display the correct content), but the prototype cell size is ignored and the collection view's item size is used instead. It's not like I'm manually setting the size. What's the point of allowing the prototype cell to be sized in storyboard if the property is just ignored when it's actually displayed? I'm using a UICollectionView with two prototype