Strength shouldn't matter that much for the armdrag. Knox is right about it being a timing issue. Some other things to keep in mind with the armdrag:
1. Get your hand HIGH on their arm, in the armpit. If you are as high on the arm as you should be, the concept of "pulling out their arm" isn't even relevant. You aren't grabbing their arm, you are grabbing their SHOULDER. Also, get your shoulder touching theirs. If you're dragging their right arm, your right shoulder should be touching his.
2. You don't need to pull them anywhere for it to work. You can pull yourself behind them instead of moving them. For much larger opponents, this is what you will need to keep in mind.
3. It works best if the guy is coming forward, or committing his weight to you. Guy reaches out and tries to pin your butterfly guard foot to the floor, or puts his hand on your knee with weight on it, armdrag that arm immediately. He is giving you his balance when he puts his weight on something that belongs to you.
For the posturing in the guard: this is a problem for all small guys. One thing that will help is a similar concept to the armdrag. When you put your weight on him, you are giving him control of your balance. Therefore, when you posture up, the hand on his midsection or belt should not have any weight on it. The hand is just there to keep him from coming up. You should be sitting back more, with your back straight, and NO weight resting on him until he tries to sit up into you.
Another thing to try: STAND UP. It only takes a second of posture to stand. Grab his right wrist/sleeve with your right hand (don't extend it, keep it tight). Post up your left foot first, stand up. If during the first part, he grabs behind your head to keep you down, use your other hand to PUSH IT UP AND OFF and posture/stand up quickly. From there, he may sweep you, but it is much harder for him to break your posture, and the sweeps can be dealt with eventually.
