Think in terms of "excess thrust" to get a feel for the vertical benefits. If your plane weighed 26 lbs and had 30 lbs of thrust, you would have 4 lbs of excess thrust - thrust additional to weight that is available to accelerate the mass upwards. Take a pound of airplane away, and now you have 5 lbs of excess thrust, which is a gain of 25%. That's why it seems like gains are better than 4%.