You're absolutely right, Canda -- the Wal-Mart on the island is "just" a regular Wal-Mart. There is a Super Wal-Mart before Hilton Head.
There are plenty of grocery options on the island itself, though -- two Publix (one near the Wal-Mart, another near Sea Pines Circle), two Harris Teeters (one at Northridge Plaza, the other near Sea Pines Circle), two Piggly Wigglys (one at Shelter Cove, near the Disney resort, the other at Coligny Plaza) and a Food Lion (across from the Publix at Sea Pines). That's just what I can think of off the top of my head. I personally tend to like Publix first, then Harris Teeter.