If the update is to existing assets, the old version will just be removed.

E.g. if they changed the colour of an object, they can just delete the old texture file and just keep the new one

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The 80GB update is replacing 80GB of the existing 100GB game. For example if they update the skin of a character the new skin would replace the old skin, there is no reason to keep both the old and new skin. So the updated file containing the skin of the character would not change size. There would still be only one skin with the same resolution there.

It's replacing data not adding new data.

But also depends on the game and system.

For example the first year of the ps4 the system couldn't insert data so if a game had a 50gb update then a week later got a 1gb update. It had to download all 51gb that week.

This changed later on but it's what caused the "copying" which took forever on ps4.

Some games like fallout 76 and kingdom come deliverance structured their files such that if a few things were updated a larger chunk also had to be reinstalled. That is why both games had day 1 patches the size of the game itself

Youre talking about Destiny 2 arent you...

Its because the way Bungie works their updates for the big expansions is you essentially re-download the entire game twice, and then it switches over to the "new game" when its done and deletes the "old game"

There are multiple ways of how to make updates.

One is to just download a list of changes and apply them locally on the existing files, but this can fail if someone tinkered with them.

Second is to just download the whole file and replace the already existing one. This takes more time and data, but it guarantees success.

In your example they choose second method. You download 80GB update and that will replace 80GB of your game.

I had to think about something while I was waiting I guess.

For the same reason that replacing the engine in a car does not make the car larger. Some games can't do binary patching because of the nature of their randomly packed data files makes the entire pack change. So they have to replace the entire data store instead of just inserting a new version of a specific file.

It overwrites the data just how saving your game on top of an existing safe file overwrites the previous one.

If the total size of the game file is a 100GB and the update is 80GB, then 80GBs worth of data is being replaced or over written with new/updated information.

Good analogy!

And it's worth noting that it doesn't mean the skin itself has to change for the file that contains it to change.

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