UPDATE: The premonition about fragmentation was correct. The morning after the pruning was filled with dropped frames. Be careful using this operation on volumes used for video storage.
When you write files to an Xsan volume, an application can ask the metadata controller to preallocate a certain number of blocks on the filesystem. But you may not end up using all those blocks for data. To quickly recover that allocated, but unused, space, type this command:
sudo snfsdefrag -rvp /Volumes/XSAN
Instead of "XSAN" type the name of your Xsan volume. On one 14 TB volume that was a bit less than half full, I recovered 700 GB. (Of course, that space is going to be pretty fragmented.)