You can free some space with:

conda clean --all

clean Remove unused packages and caches.

Conda already use symlinks when possible for packages. So, not much to improve here, I guess.

Ok, thanks, but I would like to know "not for a specific environment, but in general" - for all environments.

You can list all packages in all envs with a few lines of Python:

import os
import subprocess
for env in os.listdir('/Users/me/miniconda3/envs'):
    subprocess.call(['conda', 'list', '-n', env])

Finally I got around dealing with this issue. In the end it was a couple of days work:

1. For all my Python projects I use PyCharm and with it I checked which project uses which environment. For all environments I used the conda env export > environment.yml to save the settings of the environment from https://conda.io/projects/conda/en/latest/user-guide/tasks/manage-environments.html#exporting-the-environment-file
2. Check whether my projects still work with new environments created from the environment.yml.
3. Use anaconda-clean from option B in https://docs.anaconda.com/anaconda/install/uninstall and put the created backup in a save place.
4. Rename the old anaconda2 directory to anaconda2_backup.
5. Install a new conda environment - miniconda3 in my case.
6. Build new environments which are need for current projects from the environment.ymls and check whether these work.
7. Delete the old anaconda backups.

Finally I also reduced my logical volume with https://blog.shadypixel.com/how-to-shrink-an-lvm-volume-safely/ but this is only for Linux users using LVMs.

This way I was able to free 20 to 30 GB of space.