So maybe I was quick to comment, but I glanced at the images I have sitting around and python:3.7 is 876MB. However, python:3.7-slim is 112MB (still nearly twice the size of the image you describe, though). But it's conceivable that many people running into limitations with the slim image would just opt for the full Python image instead.

Maybe a better example is the Tensorflow image, which is close to 4GB for the GPU version. It's also the kind of thing that would be a pain to rebuild from scratch if you wanted to find a way to save space.