Bearing surface is determined by how much of the bullets sides are parallel and will bear against the bore.

Case volumn is affected by how deeply the bullets base is seated into the case. If you have two identical bullets, but one has the crimp groove closer to the base than the other, the bullet with the crimp groove closer to the base will not be seated as deeply if both are crimped in the crimp groove, but will have a greater overall length since more of the bullet will be sticking out of the case. There fore it takes up less space in the case, and volumn is increased over the bullet that has the crimp grove closer to the bullets nose, which will necessarily have more bullet seated into the case, which creates a shorter overall length.