Omnis is packed with custom resources, which is why the file size is comparatively large. I'm currently working on reducing the DL size by every MB I can chip away. That includes converting the audio formats - a time consuming process (given that every reference in-game will have to be amended) but worth it, I think, in the end.

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EDIT
Version 13 is up, it's 21.9mb smaller than version 12 :)