Size comparison between the new Samyang 135mm f/1.8 and the Sony and Sigma competition
Image courtesy Camerasize
As you can see the Samyang nearly has the same Sony GM size but it is a lot lighter. The Sigma is considerably bigger and heavier. You can also view the exact and accurate specs comparison on this BH page.
Samyang 135mm f/1.8 FE preorder at BHphoto, Adorama. Calumet DE, FotoKochDE and at WexUK.