The PictureSwiper example does just that, setting the view frame whenever the window is resized. However, you need to set layout / resizing restrictions. Whenever a view is resized, constraints will also resize NSPageController objects. If you are doing something unique with the layout / size of the view, you will need to manually resize, as in the example above.
In addition, snapshots are generated on the fly. From the arrangedObjects document, the NSPageController documentation:
The delegate will be offered photographs as needed.
And this is useful to keep in mind:
When using portrait mode, if pageController.view supports a layer, live layers are used during the transition instead of snapshots.