Alex,
looking at the wrong side of the coin; the lower V1 at the lighter weight is because of the GO case.
At the lighter weight, you can continue from a lower V1 and still get airborne in the runway remaining. As weight increases, you need to be faster (on all engine acceleration) before the engine fails so you have less accel required (on one less engine) to reach Vr.
Same scenario when reducing the runway length available, the V1 goes UP because you have less runway from Vef to accelerate to Vr and get airborne
(this is not always the case but most common, short runways and heavy weights will be stop limited)