The file hadn't been small because it lacked content. It had been small because the coder who compressed it was a genius, stripping the game down to its barest mathematical bones and writing a script to regrow them on the user's machine.
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Many fans actually prefer the original lighting and water refraction effects over the remastered version, which some argue looks "flatter". BioShock PC System Requirements The file hadn't been small because it lacked content