Why stencil buffer use memory with R8G8B8A (4byte)?

I ran memory profile on my device(memreport -full), specifically, experia sony.
But my custom stencil buffer use 32bit? I don’t know thumb of rule for size of scencil buffer, but in my experience, stencil value could be from 0 to 255. so It could be reduced as 8bit(like gray buffer) I wanna reduce of this. 3.25MB is too much for mobile.

And I’m attaching Renter Target log for memory, so could you tell me more what I can reduce?
Thanks in advance.

Pooled Render Targets:
0.001MB 1x 1 1mip(s) WhiteDummy (B8G8R8A8)

0.001MB 1x 1 1mip(s) BlackDummy (B8G8R8A8)

0.001MB 1x 1 1mip(s) BlackAlphaOneDummy (B8G8R8A8)

0.001MB 1x 1 1mip(s) GreenDummy (B8G8R8A8)

0.001MB 1x 1 1mip(s) DefaultNormal8Bit (B8G8R8A8)

0.062MB 128x 128 1mip(s) PerlinNoiseGradient (B8G8R8A8)

0.001MB 1x 1 1mip(s) MaxFP16Depth (FloatRGBA)

0.001MB 1x 1 1mip(s) DepthDummy (DepthStencil)

6.504MB 1184x 720 1mip(s) SceneColorDistorted (FloatRGBA) <<< what is this?

3.252MB 1184x 720 1mip(s) SceneDepthZ (DepthStencil)

3.252MB 1184x 720 1mip(s) CustomDepth (DepthStencil)

3.252MB 1184x 720 1mip(s) MobileCustomStencil (B8G8R8A8) <<<<

0.407MB 296x 180 1mip(s) BloomSetup (FloatRGBA)

0.103MB 148x 90 1mip(s) BloomDown (FloatRGBA)

0.026MB 74x 45 1mip(s) BloomDown (FloatRGBA)

0.007MB 37x 22 1mip(s) BloomDown (FloatRGBA)

0.002MB 18x 11 1mip(s) BloomDown (FloatRGBA)

0.007MB 37x 22 1mip(s) BloomUp (FloatRGBA)

0.026MB 74x 45 1mip(s) BloomUp (FloatRGBA)

0.103MB 148x 90 1mip(s) BloomUp (FloatRGBA)

0.407MB 296x 180 1mip(s) SunMerge (FloatRGBA)

16.000MB 2048x2048 1mip(s) WholeSceneShadowmap (ShadowDepth) << I’m struggling with reducing
resolution this too.

16.000MB 2048x2048 1mip(s) OptionalShadowDepthColor (B8G8R8A8) << same as above

6.504MB 1184x 720 1mip(s) SceneColorDistorted (FloatRGBA) <<< second time for sceneColorDistorted

3.252MB 1184x 720 1mip(s) Tonemap (B8G8R8A8)

1.000MB 128x 128cube 8mip(s) ReflectionColorScratchCubemap0 (FloatRGBA)

1.000MB 128x 128cube 8mip(s) ReflectionColorScratchCubemap1 (FloatRGBA)

0.062MB 32x 32cube 6mip(s) DiffuseIrradianceScratchCubemap0 (FloatRGBA)

0.062MB 32x 32cube 6mip(s) DiffuseIrradianceScratchCubemap1 (FloatRGBA)

0.001MB 9x 1 1mip(s) SkySHIrradianceMap (FloatRGBA)

0.001MB 1x 1 1mip(s) ReflectionBrightness (FloatRGBA)

0.375MB 296x 166 1mip(s) SunMerge (FloatRGBA)

0.094MB 148x 83 1mip(s) BloomDown (FloatRGBA)

0.023MB 74x 41 1mip(s) BloomDown (FloatRGBA)

0.006MB 37x 20 1mip(s) BloomDown (FloatRGBA)

0.002MB 18x 10 1mip(s) BloomDown (FloatRGBA)

0.006MB 37x 20 1mip(s) BloomUp (FloatRGBA)

0.023MB 74x 41 1mip(s) BloomUp (FloatRGBA)

0.094MB 148x 83 1mip(s) BloomUp (FloatRGBA)

0.375MB 296x 166 1mip(s) SunMerge (FloatRGBA)

62.298MB total, 34.455MB used, 40 render targets