Quote from eusdaiki:
It's probably not someone who's trying to buy/sell large size... since no one moves large size upfront without trying to hide it a bit.
Sounds more like someone trying to do the very illegal and very risky tactic of spoofing the market... post a huge buy order so that your smaller sell orders get filled faster at better prices...
Do you have any more details?... how big the size is in relation to the rest of the stock... does the side show up on one side and then show up on the other side after the stock has moved or a few moments later... does it vanish when trades start printing against it?