Is iCloud Document Sync "delta sync" or "full file sync"?
Delta. It splits this intelligently into chunks and only updates the relevant chunks using Rabin fingerprinting.
You can see this in Apple's WWDC iCloud presentation, and slides.
A pretty good description of this chunking can be seen in Siracusa's Lion review, however the review describes this process in terms of document versioning, but the fundamentals are the same.