If bloat is a primary concern I suspect you’d be better using jupyter notebook which opens a slightly lighter weight interface. You still pay the cost of a browser tab (up to 200 MB of you use say Firefox) but in general the notebook is very responsive.
The whole notebook interface is really best for highly exploratory work where you want to see the effect of each command. Especially it helps if you generate a lot of plots.
For light weight I think your best options are Vim or Sublime Text (though I have less experience with this option). If you want to setup Sublime here’s a post discussing it
I personally prefer the more fully featured IDEs like IntelliJ because I’ve adjusted my workflow to accommodate them. Atom is okay for me, but I do agree there’s room for improvement.