I have a scroll view that I want to be able to compress or expand based on the device. Right now I can get the page to look okay in iPhone & 3.5" Retina OR 4" Retina but not all three. I have tried playing with Content Hugging Priority and Content Compression Resistance Priority as well as mucking with all the related constraints. So far NOTHING seems to have an affect on layout. Does iOS ignore all those settings? It's driving me crazy trying to figure this out.
You need to ensure there are "springs" or "struts" from one side of the view to the other. For example, against a toolbar, Xcode doesn't indicate a default spacing (10 points), you have to "butte up" against, then fine tune to a default spacing. All spacings need to be accounted for, and if you leave one item with flexible height (e.g. height is lowest priority), such as a scroll view, then that item will be expanded or compressed as required. The debug output when interacting with the elements sometimes generates spacing constraint violation warnings, these can be helpful. CHP and CCRP don't seem to have much affect in iOS.