The GTA Online Heists update is a sizeable 4.8GBon Xbox One it seems.
Officially launching tomorrow, it seems like some gamers have managed to download the update overnight, the the Xbox 360 patch a smaller 1.2GB.
The update adds the long awaited Heist mode to multiplayer, where you and your friends and can plan and pull off various raids across Los Santos, as well as new weapons, vehicles and costumes to look stylish as you do so.