More than likely the 18GB download is an compressed archive, which may unzip to 18+ GB, and until which the archive is uncompressed parallel to the extract, the archive will not be deleted by the system (cleanup). So you may need in theory, 18GB + 18GB+ or 36GB minimum; and I'd not be surprised if was in excess of 25GB depending on the game, or 18+25 = 43GB minimum.
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