After thinking carefully over the past several days, we've decided to delay the ATLAS launch until December 19th to give the team more time to run through this massive game's extensive content.
It's a vast virtual world out there across the ATLAS, and the team's going to use that extra time to review every portion of it thoroughly. We're sorry about the delay, and we know how much everyone is looking forward to establishing their empires! ...
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The main reason ATLAS is 100 GB (other than the massive scope of the content and the large number of terrains) is that the textures are 4k resolution. In ARK they were 2k.
Ultimately we'd like to separate the 4k textures into an HD texture pack, but it's faster for Early Access iteration to keep it as a single build for now.