This issue has been closed as 'Won't Fix' due to an extended period of time without updates. If this issue is important to you please let us know by posting on the AnswerHub or UDN, and Epic will re-open the ticket for further review.
When you have a Material with a static parameter and an Instance of that Material that has the "override" checkbox checked for that static parameter, but has the value set to the same value as the parent Material, even though the resulting shader code will be the same for the Material and it's Instance, during cooking a copy of the shader is created for the Material Instance, wasting space and memory.
The same thing can happen when checking the Blend Mode override checkbox in the material property overrides of the material instance, but then setting the blend mode to the same as the parent (for example overriding a masked blend mode material and setting the instance to be masked as well). In this case, the cooked material instance is not only duplicating code, but is actually somehow larger that the parent material, even though it should be an identical shader.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-20883 in the post.