It is where the weight is, and the weight added needs to make the machine stiffer.
Just adding weight to the base probably won't make the column any stiffer.
All the stiffness is required to react the force of the cutter between the spindle and the job. Once the spindle/column/cutter deflect more than the chip thickness (and I don't mean depth of cut) you get chatter and a poor surface finish, as the whole assembly oscillates.
Increasing the torsional stiffness of a column can help, but it won't be that simple. If you thump the head with your hand and feel vibration, then the vibration is an indication of lack of stiffness.