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ScriptableRenderPipeline/HDRenderPipeline/HDRP/HDRenderPipeline.cs

             {
                 Vector2 pyramidScale = m_BufferPyramid.GetPyramidToScreenScale(hdCamera);
 
+                // Need to account for the fact that the gaussian pyramid is actually rendered inside the camera viewport in a square texture so we mutiply by the PyramidToScreen scale
                 var size = new Vector4(hdCamera.screenSize.x, hdCamera.screenSize.y, pyramidScale.x / hdCamera.screenSize.x, pyramidScale.y / hdCamera.screenSize.y);
                 uint x, y, z;
                 m_applyDistortionCS.GetKernelThreadGroupSizes(m_applyDistortionKernel, out x, out y, out z);

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/Lit/Lit.hlsl

 TEXTURE2D(_PyramidDepthTexture);
 
 CBUFFER_START(UnityGaussianPyramidParameters)
-float4 _GaussianPyramidColorMipSize; // z = lodCount
+float4 _GaussianPyramidColorMipSize; // (x,y) = PyramidToScreenScale, z = lodCount
 float4 _PyramidDepthMipSize;
 CBUFFER_END