We had optimized it previously for the iPhone and iPad displays, not for the 6 and 6+ resolutions. So we're working on that but it'll mean having to update our entire database and that may take time. It's on the list of things to do though.
Tags are not as prevalent on Windows, primarily because there's not quite as much that we can do with them. On OS X Apple provides some really great features to let us use tags in unique ways without having to recreate the wheel. It's nearly free in the end for us as developers to do tags and smart folders.
On Windows though we would have to completely re-implement all of the features for tags and smart folders from the ground up to do what we do on Mac. At the moment Smart Folders do not exist at all but tags do, they're just hidden.
If you click in the search field of the Windows application, then press Alt+Down Arrow you should see a list of tags. These are items that were tagged on Mac and iOS, but I'll see what we can do about bumping the priority on this so that when we get to new features again we'll hopefully take a better look at it.