Your mattress should be just firm enough to ensure that when lying in bed your spine is level or in a 'neutral' position. Whether you are a side, back or front sleeper, your mattress should support or cradle you in this level position.
The heavier you are, the firmer your mattress should be, or if you are a side sleeper your mattress should be soft or forgiving enough to contour to your side profile.
To get the correct support at the main contact or pressure points - your neck, shoulders, lower back, bottom and hips - your body weight needs to engage the springs.
A light person on a firm mattress is not heavy enough to engage the springs, so the spine will be raised at the hips and shoulders. A heavier person will not get sufficient support on a soft mattress, dipping into it too far.