Let the diameter in consideration be AB as in the above figure. The surface to the right of AB pulls the surface on the left of AB (say by a total force = F) due to surface tension. The semicircular walls of the tube in contact with the surface to the left of AB also pulls it towards left with a net force F. Thus the semicircular surface ABD is pulled by two equal and opposite forces, making the net force on it zero. So it remains in equilibrium.