That's not how compression would work. Think about if they are adding a new level. The algorithm would have nothing to compare the files against except what is actually being sent. If the texture appears in each level, and each level comes in a separate update, there's no way for the algorithm to reference or incorporate the files that are already in your computer, so it has to compress it like it's a brand new file. That is unless you have a costume installer which duplicates the file once it runs, but that would probably be a ton more work for the developers.
You have this completely wrong. "there's no way"? It's extremely easy to make a patch file that references existing data. This is a problem that has been solved many times.
And that's not relevant to initial install anyway, which can/should be a single download.
Having a reference dictionary is a type of compression.
You need a program that decompresses no matter what, and the ability to reference existing files barely changes it. It's not a post-processing stage where files get copied around. It's the ability to reference arbitrary chunks of data inter-file just like it can reference arbitrary chunks of data intra-file.
At the most basic level it's like taking a compressed file and chopping it in half. The user already has the first half, then they download the second half and "resume" decompressing.
