In my opinion I have found that I can spray a straighter line with a heavier airbrush than a lighter one. The weight of the heavier AB absorbs small movements. Think of it with a car analogy. On a straight highway a small, lighter, car jiggles on bumps on the road and a larger, heavier, car glides across the same road. Although a heavier airbrush might seem slightly heavier, in action, that airbrush has momentium, a built in shock absorber in its weight, and will help draw a straighter line.
You just have to get used to a heavier airbrush. It only takes a minute.