On Windows Phone 8 the updated map data can be downloaded by heading into Settings -> Applications -> Maps on your Windows Phone 8 device and choosing the "check for updates" option. The phone will check for updates for maps that you already have installed on your device, and then ask you whether you want to proceed with the update. The download size will vary depending on what maps you already have installed.
Nokia has also been quietly working on improving the map update process on Windows Phone 8. Part of this is about optimising server performance for the delivery of large file size. However, more notable is the fact that Windows Phone 8 GDR2 introduces the ability to download only the data that has been changed, rather than the full data file for each country.
For example, with the latest update the download size for the United Kingdom is 295.9MB on the Lumia 920 (running Windows Phone 8 GDR1), but only 61MB on the Lumia 925 (running Windows Phone GDR2).