Confusion of using size classes in xcode 6

Question

I'm new in using size classes and am so confused about it. In a new storyboard project, the size classes checkbox is defaultly checked and set to wAny hAny. I right clicked the main.storyboard and see the source code,I see "width="600" height="600"",which is as expected. then I switch back to interface builder,and change it to wCompact hRegular,and look into the source code,it still shows width="600" height="600", and that confused me.Shouldn't it be width="400" height="800"? Is there something i missed? Thank you advance.