It's been revealed by EA DICE that the day one patch for Battlefield 3 on the Xbox 360 is going to be quite hefty - coming in at around 167MB.
The patch will have to be loaded up as soon as the game is connected to the internet. If you aren't connected, you'll be able to jump straight into the single-player campaign as normal.
EA hasn't announced what the patch does though, and there's no word about whether similar patches will be available on other platforms. Either way, this is a huge update.