It's not just UNIX. The cluster size (block size in UNIX) is the smallest unit of size any file system can reference when accessing disk storage.
replies(1):
“Block suballocation is a feature of some computer file systems which allows large blocks or allocation units to be used while making efficient use of empty space at the end of large files, space which would otherwise be lost for other use to internal fragmentation.
In file systems that don't support fragments, this feature is also called tail merging or tail packing because it is commonly done by packing the "tail", or last partial block, of multiple files into a single block.
As of 2015, the most widely used read-write file systems with support for block suballocation are Btrfs and FreeBSD UFS2[4] (where it is called "block level fragmentation"). ReiserFS and Reiser4 also support tail packing.”
Also (same article):
“Several read-only file systems do not use blocks at all and are thus implicitly using space as efficiently as suballocating file systems; such file systems double as archive formats.”
There also are filesystems that store the content of very small files, together with file metadata, not in blocks allocated for the file.