Here is a size and feature comparison fo the recently announced Nikon Nikkor Z 135mm F1.8 S Plena ($2499) lens with main competitors in the mirrorless world: Sony FE 135mm F1.8 GM ($2,099)and Canon RF 135mm F1.8 L IS USM ($2,099), followed by a feature and specs comparison table.
As seen on the scaled size comparison, the Nikkor 135mm Plena is the largest and heaviest of the bunch but not by much. If the claimed optical performance of the Plena is true, this would easily justify the size difference.
(click on the image for larger version)
(For further comparison of the Plena with the Nikon 58mm f0.95 Noct, check: https://cameradecision.com/blog/Nikon-135mm-F18-Plena-vs-58mm-F095-Noct-Lens-Sizes-Comparison)
Nikon Nikkor Z 135mm F1.8 S Plena vs Canon RF 135mm F1.8 L IS USM vs Sony FE 135mm F1.8 GM Specs and Feature Comparison