Scale the image uniformly (maintain the image’s aspect ratio) so that both dimensions (width and height) of the image will be equal to or larger than the corresponding dimension of the view (minus padding).

Official Docs

When the image matches the proportions of the container:

http://i.stack.imgur.com/1se4G.png

When the image is wider than the container it will expand it to the bigger size (in this case height) and adjust the width of the image without changing it’s proportions, causing it to crop.

http://i.stack.imgur.com/81ohC.png