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The obvious precedent here is Final Fantasy 7 Remake, which also came with two discs and a whopper of a drive footprint at 100GB.
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One way or another, your hard drive's getting both barrels. Instead, Final Fantasy 7 Rebirth will seemingly put more emphasis on installing content rather than downloading it to supplement what's on the disc, but that does not mean the game will be small altogether. Hell, a few physical games have actually come with a worthless decorative disc paired with a digital voucher. Why might a game need two discs? Well, we could be looking at the opposite approach that some games take nowadays by essentially including a cosmetic disc leashed to a mammoth mandatory download.
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