We're intelligent and social, but that certainly hasn't halted our aggressive behavior. I think what you are seeing is typical pecking order behavior, and from what I understand, it is a very common behavior for NA wood turtles. The fact that it is your larger individual initiating the aggression goes even further toward this. It sounds like they are both too small/young for this to be a sexually motivated territorial dispute, so you may simply have a turtle with a particularly aggressive (anti-social) personality. I would keep a very close eye on the situation and be ready to separate the two into private enclosures. In the meantime, any cover you can provide in their current enclosure may help provide refuge to the smaller turtle and help calm the tension. I think you'll need to separate them though?