For every new major version, PhpStorm creates new folders for caches and settings. And despite having a successful upgrade, the old folders won’t be removed automatically. This behavior uses much unnecessary disk space.
The most used disk space I could found is dedicated to the caches, mainly the index of all projects. Since the index is renewed for every project after every major update, the old index is useless. For me, it has a size between 3,6 GiB and over 9 GiB – per major version!
You can find the caches in ~/Library/Caches/JetBrains ordered by major version. All folders with outdated version can be deleted here.
The second directory is dedicated to the settings. Even if they’re much smaller (in my case around 95 MiB), the old ones are still useless. You can delete them in ~/Library/Application Support/JetBrains.