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Poisson's ratio is the lateral contraction divided by the axial extension, so 180 microstrain of narrowing under 600 microstrain of stretch gives 0.30 - the standard steel value. It caps at 0.5, where the material conserves volume exactly, and rubber sits close to that. The volumetric strain shown is epsilon(1-2nu), which vanishes at 0.5.
Poisson's ratio
nu = lateral strain / axial strain
Volume change
volumetric strain = epsilon (1 - 2 nu)
Beyond that the material would gain volume when compressed, which violates energy stability for an isotropic solid.
Yes - auxetic foams and some engineered lattices get thicker when stretched. They are rare and deliberately structured.