changing tablespace seems to waste space
The comment about file system block size may be relevant. Please note that it is about file system blocks, not hard disk blocks. This is normally referred as a cluster, in NTFS terminology. You may check it with command fsutil fsinfo ntfsinfo [your drive] on a cmd opened as Administrator.

You might also check the storage parameter, like fillfactor. I think (althought I am not sure) that moving a table to a new tablespace will not just copy tha table blocks, but will reorder them. If you copy a table where you never issued update or delete statements, the table on the new tablespace will be as the original one, but if you updated/deleted records, the the table on the new tablespace will be "compacted" and will fill all new blocks only until the fillfactor, leaving some space for future updates on every block. If the fillfactor is too large, this is a waste of disk space.