This preloading is currently only available for Xbox One owners. PS4 and PC owners will surely get a preloading period on their platform before Star Wars Battlefront is released.
As for how big of a game Star Wars Battlefront is, the file size is surprisingly small. Coming in at 19.4 GB, Star Wars Battlefront shouldn’t require too much hard drive maintenance to download. The reason for the small file size is almost definitely because of a lack of single-player.