As with all electronics, various configurations of parts can mean the product will be more or less "efficient". If two vibes take the same batteries in the same quantity, the more efficient vibe will, as you mentioned, be stronger because it is taking the same input to make a greater output (work being done = motor being turned). I'm not going to get into an electrical engineering thing, though.
The design of the vibrator also comes into play. If the toy is soft, squishy, and/or well insulated, the vibrations will be more diffuse and feel weaker. A smaller, hard, pinpoint vibe with the same power source will put out stronger vibrations than a large vibrator with the same size motor, hidden underneath a bunch of jelly.