Why is the python docker image so big (~750 MB)?

You can try the python:{version}-alpine version. It’s much smaller:

>> docker image ls |grep python
python    3.6-alpine     89.4 MB
python    3.6            689 MB
python    3.5            689 MB
python    3.5.2          687 MB
python    3.4            833 MB
python    2.7            676 MB

At time of writing it looks like the official image supports -alpine on all python versions.

https://hub.docker.com/_/python/