While you might find files online claiming to be 10 GB or smaller, downloading from unofficial sources carries severe risks: How to download and play Diablo IV on PC (Windows 11)

Diablo 4 requires a constant internet connection to Blizzard’s servers. Even the single-player campaign is hosted server-side. A cracked, offline "compressed" version cannot work because the game logic, enemy AI, and loot drops are processed on Blizzard’s machines, not your PC.