https://wccftech.com/world-of-warcraft-ptr-size/

Blizzard has rolled out a World of Warcraft PTR update that significantly reduces the download size and install size of the PTR when World of Warcraft is installed.

The update brings the download and install size of the World of Warcraft Public Test Realm to 2.64GB. In order to take advantage of this new patch, players with the PTR installed will need to perform an uninstall and reinstall first. We’ve included the release notes for this new build down below: