Actually it was the impulse engines that reduced the ship's mass -- otherwise it would've taken weeks to accelerate a vessel that size to a velocity suitable for interplanetary travel, and a comparable time to decelerate. Think about how oil supertankers need hundreds of miles to slow to a stop or even to make a turn because of their immense mass and momentum.
However, the impulse engines used a low-level (below 1000 millicochranes) warp field to achieve the mass reduction. After all, mass and spacetime distortion are essentially the same thing, so the best way to cancel out an object's mass would be to "flatten" the spacetime around it. And a warp field is what you use to bend spacetime.