Sativas are generally tall and thin, but they don't start getting really noticeable, til they get a little bigger. Usually, they really go crazy once they start flowering, not so much, beforehand. Are you using one of those moisture meter thingys? Those really aren't that dependable, and will give you problems, like you have right now. Your soil could be dry around the sides, top, and bottom, yet the meter will tell you that everything is okay, because there's still moisture in the center, where the probe is at. When, in reality, all the roots that are sitting in those dry areas, aren't taking in anything, food or water, starving the plant, effectively making it think it's in a smaller pot, keeping it confined. Watering, is such a hard thing to decribe, as far as when to do it, and why. What I can tell you, is that if you soil is dry around the edges, and the top is dry(an inch or so), that you don't have to worry about overwatering. Overwatering, is keeping the roots soaked to the point that they are deprived of oxygen, which is very wet, nowhere near what you have now. Cannabis plants love water, but they love oxygen, as well. It's finding that balance, that's the key to success. What I've found to work best, is to water heavily(so that every single grain of soil is wet), then let them dry for a few days, to about where your's is now, then repeat, over and over. Same thing when you feed. It's foolproof, and guaranteed to work, as long as you let them dry out afterwards, there's no threat of overwatering, at all, unless you have shitty soil with poor drainage, in which case, you do not.
