I'm not a pro drag racer, but have been around it all of my life.

Another thought that occurs to me... besides simply increasing power... is that the tires are gripping the asphalt, trying to accelerate the car's mass. But as weight increases, you may approach or surpass the limit of the tires "gripping" ability.

...of course there may be the option of bigger tires... :-)

Other advantages to light weight are the decreased stress on the drivetrain, and the potential to go even faster, when the power is increased... as you did in the heavier example.