Memory optimize

when i use “Memreport -Full -Log” and see log, i found render target use a lot of memory.

16.000MB 2048x2048 1mip(s) WholeSceneShadowmap (ShadowDepth)

16.000MB 2048x2048 1mip(s) OptionalShadowDepthColor (B8G8R8A8)

how to optimize the memory? is there any way to decrease size of it ?