Having detected an engine failure at V1 it must be possible to:
A. Reject the take-off and bring the aircraft to a full stop within the remaining ASDA.
B. Continue the take-off with one engine inoperative and reach V2 at screen height before the end of the TODA.
Reducing V1 reduces the distance that is required to accelerate up to V1 and also reduces the distance that is required to decelerate to a full stop when rejecting the take-off. So reducing V1 reduces the distance required to achieve option A. This means that reducing V1 decreases the acceleration-stop distance required. Increasing V1 will have the opposite effect.
The key factor in option B is that after engine failure at V1 the remaining acceleration to V2 within the TODA, must be done with one engine inoperative. Reducing V1 increases the speed difference between V1 and V2. This increases distance that is required to achieve this speed increase. So reducing V1 increases the take-off distance required (TODR). Increasing V1 will have the opposite effect.