By “light”, “medium”, and “heavy” I am talking about how much disk space they will occupy. I should clarify that in the text. Thanks for the feedback.
From a CPU / GPU perspective, Lubuntu and Xubuntu (based on lxde and xfce, respectively) are quite light weight.
However, installing them will add over 1GB to your hard disk because they install more than just the GUI, they also install many, many applications.