All of this talk about watering and no guidelines for amount of water so you can keep it under control. With larger pots now, your soil will hold exponentially more water, so fight the urge to wet the top layer and fill the drip trays and crap like that every time you enter the room. Let your pot dry out. Let me say that again, let your pot dry out until you have that nervous feeling in your stomach, then add only enough water to your pot until it BEGINS to run to the drip tray. Let it dry again. Figure out how many days it took to dry out in between waterings then make yourself a schedule that you can stick too. This process takes discipline or every idiot would be doing it. A good rule from Soma and Jorge is to stick your finger in inch or 2 into the medium, if it feels like there is moisture, don't water. You must have abundant oxygen in the root zone to stimulate growth, and every time you water you displace that oxygen in the medium, causing the plant to suffocate a bit. Back off on the watering and you will notice dramatic changes very quickly.

Also, I'm shocked to see that no one has mentioned that you are vegging under an HPS lamp. HPS lamps have great lumens/watt and plenty of light for growth, but unfortunately the plants light requirements change with growth, and HPS should be used for flowering. HPS is way in the red spectrum on light, simulating autumn in most places, about 2700K or so. Great for flowering. But when you are vegging the plant needs blues also which your plants aren't getting. You should run a lamp with at least a 4000K rating to get more of the proper light for vegging. Consider getting a conversion bulb to run a Metal Halide on your HPS ballast, or getting a switchable ballast that has both types. I would say though, if you are only going to run one type of lamp for both stages of growth, a MH would be much better for you.