2 OS X automatically works out what the best amount of space is for the apps you combine in Split View. When you combine Mail and Reminders, only a narrow strip is allocated to the latter app. Just as OS X recognises that Reminders only needs a small amount of space, it knows the minimum required width of each app. So, iBooks and Pages can’t be put side by side on a 15-inch MacBook Pro, yet they can be combined on an external, higher resolution display.
On a MacBook with a Retina display, you can sometimes overcome this limitation. In System Preferences > Displays > Display, click Scaled and choose an option further towards the ‘More Space’ end of the range – this causes everything to be rendered smaller on the screen, yet some apps provide a zoom control, so you can keep what you’re working on comfortably large. If you use this technique often, consider buying EasyRes (£5.99, Mac App Store), which enables you to quickly switch resolutions from the menu bar.