Stretching can affect girth gain?
When a sample of material is stretched in one direction, it tends to contract (or rarely, expand) in the other two directions.
“Poisson’s ratio “(PR), named after Simeon Poisson is the ratio of transverse deformation strain to longitudinal deformation strain in the direction of stretching force. “Strain” is defined in elementary form as the change in length divided by the original length.
Since, in stretching exercises either by hands or weights, the forces on the penile tissues are almost axial (longitudinal) so there should be contraction laterally in the stretched tissues. If one does only stretching exercises over a long time and gets some gain in length, his should lose his girth by a little amount. However, in girth increasing exercises, no pure one-directional forces are induced in the penis. In fact forces are applied in all directions by combination of hands and blood pressure in jelqing and mostly by blood pressure in clamping. So, Poisson ratio affect cannot be applied in these cases. So pure girth exercises should not affect length at all.
So, here is my conclusion, length gain by stretching and weights should affect girth gain a little, but reverse case should not occur. My idea may not be correct, but I have tried to cast something using the concept of my penile enlargement incorporating with properties of materials.