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