Even though people were surprised at the beginning of the current generation by the huge size of the most ambitious triple-A games, that has now become the norm and players are now used to putting aside around 40 GB of disk space on their consoles for the more anticipated titles.
Thus, it's news when a high-profile project halves those sizes. That is the case with the upcoming Rainbow Six: Siege, which will take just 16.54 GB of space your on Xbox One hard drive, as confirmed on the official listing for the digital version of the game.
Keeping in mind that it's a multiplayer-focussed experience with no single-player campaign (although there are Situations), and that we're of course not talking about an open-world game, it might just be because of really good optimisation, or perhaps there's some recycling of assets.
Similar file sizes are expected for the PC and PS4 versions of the game, all of which are expected to arrive on December 1.