With every part, we try to save as much weight as possible while retaining its structural strength. Finite-element analysis helps to understand the exact strain on the part. Material can be taken away where it is not needed.
The shown part is a gear wheel of about 2 cm width. (Thats about 3/4th of an inch) The FEM analysis shows that the material could be turned down to as little as 2 mm without overloading. Some security factors must be added, so the final part will be about 3 mm.
In this case, the weight can be reduced to less then 50% of the original weight.