V1 is the accelerate/stop distance.Speed to decide which on"limiting" runways(always assumed).With Heavier weights, V1 higher(engine out go=stopping distance)once the failure is recognized after Vef.Runway annalysis charts predicated on set runway distance,should a runway be changed the crew are required to confirm sufficient runway exists!!!!Remember"flex/reduced"thrust utilizes all the runway as stipulated on the Chart.The higher the weights the higher the V1's to meet the stop or go.