What information does log_civicrm_group keep? How is it useful?
There had been an issue recently where way more 'changes' where logged than intended: https://lab.civicrm.org/dev/core/issues/449.

Can you verify that it is still growing way too fast since running on a patched version?

The information you are looking for is stored in the table civicrm_subscription_history aswell as civicrm_group_contact for current status.

With log_civicrm_group you could track how the title, description or hierarchy of a group change over time.