Obviously faced with the necessity to repartition and restore everything, I remembered from university 20 years ago that Unix file systems use one disk sector (512 bytes) for every 13 inodes or so, eating lots of disk space. A quick man newfs revealed a default of a 2048 bytes block size per inode. So I ran this command:
newfs -i 4096 yourswapdevicehere
This saved half of the wasted space, freeing up the space I wanted.
Notes:
- This will help on any UFS volume, unless you have tons of very small files to store that are better accommodated by smaller block sizes.
- Memory is also handled in page sizes of 4096 byte, so this should give a slight performance boost when used on swap volumes.
- Extended HFS files systems also utilize a block size of 4096 bytes by default.

