ePubs are a universal file type, so they aren't formatted specifically for any one device. A single ePub file is designed to work across all devices and apps used by Amazon, Barnes & Noble, Kobo, and other retailers.
The dropdown menu above the previewer allows you to switch the preview to Print or select from a variety of digital reading device preview options. These are not exact, nor universal, but they will give you a good representation of your book in various devices.