With switching to say a different plastic. The new material likely has a higher density then the previous type of plastic. Would be one example if increase. Now with the new material they can reduce wall thickness & still have the same durability.
If your Familar with Back to the Future; The Dolorien car is made of Stainless Steel which is stronger & heavier than steel used in regular cars. So if you created a steel replica with the same mfgr specs it would be lighter & less durable.
But there could be other factors increasing weight. New optics may weigh more than v2 & v3 & other components that may not have been present in previous versions.
This is a guess based on what @xunshu reported.