You can choose to compress files on storage. If on linux. Or use tools like ConpactGUI to compress in windows 10/11.(not recommended to compress live service titles)

Part of the reason for the bulk of course is prerendered video and voice assets, especially if it has multiple voice options. Also non standardization of os level compression means you cannot send those conpressed files for users as not all users could use them if compressed using the methods mentioned above.