GPU Weird Memory Usage

I have a quick question.

I am new to GPU rendering, I have an AMD RX570 GPU with 8GB of VRAM and an AMD 2700X CPU.

I don’t know if this is strange or what but I am getting faster render with my CPU than on GPU, not only that but when I render on GPU it uses a lot more memory for the scene compared to CPU (see the image below).

Is that normal? the only thing in the scene is a UV sphere and a sun lamp, I don’t see why it should consume so much memory. Any helpful explanations?