How to see sizes of installed pip packages?

Modified for pip version 18 and above:

pip list \
  | tail -n +3 \
  | awk '{print $1}' \
  | xargs pip show \
  | grep -E 'Location:|Name:' \
  | cut -d ' ' -f 2 \
  | paste -d ' ' - - \
  | awk '{print $2 "/" tolower($1)}' \
  | xargs du -sh 2> /dev/null \
  | sort -hr

This command shows pip packages, sorted by descending order of sizes.