The launcher shows the full size of the game when updating. As it checks over existing files when updating and only downloads the new changed content.
Typically updates that involve things like new meshes, textures etc are larger since those file changes take up a lot more data.
Assets are frequently bundled and located in precise ways to improve load times or otherwise optimize performance. This means that if a little detail of a level changes it may be in the middle of a bigger block of stuff.
"The file size of an update is not evidence of the amount of carelessness or mistakes made in original releases. You can have a massive bug fixed in 2 kilobytes, or 15 gigabytes of small quality of life fixes." - Rami Ismail