While magical 100MB downloads don't exist, there is a legitimate way to save space when archiving Xbox 360 games:

While the download is faster, the time to extract the file (decompress it) can be significant, depending on your computer's speed.

The most popular method, where the game data is extracted from the ISO and packed, leaving behind non-essential data.

The monitor didn't flicker. Instead, the room did. The shadows in the corner of his office seemed to deepen, losing their resolution, turning into jagged, low-poly blocks. A familiar chime—the iconic Xbox 360 startup sound—echoed, but it was pitched down, slowed to a tectonic crawl.

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may be less than 200MB, but its ISO will still show as 7GB due to padding. Removing this padding is the only legitimate way to "highly compress" these specific titles. Format Matters : Converting an ISO to GOD (Games on Demand)