It's trying to put a size comparison so that our human minds can picture the scale. Earth is just a placeholder in this example. The author is inviting you to p

It's trying to put a size comparison so that our human minds can picture the scale. Earth is just a placeholder in this example. The author is inviting you to picture the the size of the planet. They could've just written the sizes in terms of 10^x , however that is not as human-friendly to envison. There is no need to be unkind towards the author here, maybe your point is that they could have done a better job at presenting the scale for the reader? How would you write it?