"It probably means nothing..."   

It is a way that the MMs communicate with those watching the ticker.  See repost below-

Repost- You'll notice that the bid and ask have higher volumes (500K and 250K?) than typical bid and ask at those price points.  The MMs are giving us a bird's eye view of the market so that if someone wants to step up and buy or sell a big chunk of shares they can see where the market is at those volumes.