Through what I can only assume is magic, the latest update for Forza Motorsport reduces the install size by up to 29GB. Specifically, the release of Update 7 reduces the total file size by 25GB on Xbox Series X|S and by 29GB on PC. This is a size reduction on about 15-20% depending on which platform you play on.
Developer Turn 10 says that due to the nature of this new optimization, today's Update 7 will be "slightly larger"...