As I understand that, ignition is not the same as thrust. It is planned to thrust earlier then as used now.
As a side effect, the weight of the fuel needed for that ignition to thrust time, can also be saved and thus the sizes of the tanks can be shrinked a little bit, too.
Other parts that can be optimized is the structure by itself. They measured tensions of material and have been oversuprised, when the rocket did not break in the horizontal while still thrusting. That would mean, some parts could be reduced in thickness/Material.. its not a usual flight behavior for the rocket to be in the horizontal, so a reduction can be done. Just the weight, that is needed for the vehicle to perform the most needs in terms of withstanding the forces during a regular flight maneuvering.
And then, it's possible to reduce a booster as it's may be not needed anymore.
The guys doing funtastic work!
Edit:
Also, three of the drivers haven't been used, as said, on purpose. So. Still thrusting enough.. reduce 'em three!
