A question arose on Pricescope about why an 8-8.5mm stud feels so much heavier than a 7.5mm stud.
Besides the volume increase, there is the issue of torque. Torque is rotational force. The farther a weight is from the axis of rotation, the greater the rotational force it exerts. Larger studs, being spheres, protrude outward from the earlobe farther than smaller ones do. Any increase in pearl diameter increases the distance of the farthest-most part of the pearl from the axis of rotation (i.e. the earlobe) and therefore increases torque.
Gravity pulls downward on all studs, but the rotational force is greater when the stud protrudes farther out from the ear. This is why larger spherical studs are much more likely to sag.
Button shaped pearls that are the same weight as a spherical pearl won't sag as much because the rotational force is less. Simple physics.