Why does a shaft subject to pure torsion show an increased diameter in the deformed result?  See the attached example.  A shaft is fixed on one end with a torque applied at the other.  The displacement plot is the UY direction along axis 1 (for tangential displacement).  This plot makes sense as it shows the most twist at the outer shell near the end of the shaft & basically no twist at the centerline & base of the shaft.  The plot is shown both with & without the deformed shape.  Why does it show this large increase in diameter?