I think what they are saying is the backup is way less than your storage on the phone. My iPhone is using 280 GB's of storage, but the backup is only like 4 GB's on iCloud. I do have like 80 GB's of data and documents backed up as well though.
To expand on the other answers, your iPad doesn't have to transfer 512GB to sync. Only things that have changed have to be transferred and that will always be much less than the size of your storage. Not only that, it's automatic, and happens without requiring your time or attention.
Nope. That would mean managing content all the time. Not my idea of fun and I'd find it way too inconvenient.
As others have said, the transfer speeds with new devices is pretty high over USB and every later sync or backup is incremental and only touching new files/ changes.For those who use 512GB iPad and have a lot of data, how do you handle the backup? I guess syncing between iPad and your computer will take very long ass time.
And is there a way you can sync your iPad with your home cloud via Airport?