With regular compress mount option, the image file did not compress (111GB on disk):
Processed 1 file, 16992 regular extents (16992 refs), 0 inline.
Type Perc Disk Usage Uncompressed Referenced
TOTAL 99% 111G 112G 112G
none 100% 111G 111G 111G
zstd 35% 85M 242M 242M
However with compress-force, compression finally kicked in (59GB on disk):
Processed 1 file, 784669 regular extents (784669 refs), 0 inline.
Type Perc Disk Usage Uncompressed Referenced
TOTAL 55% 59G 108G 108G
none 100% 25G 25G 25G
zstd 41% 34G 83G 83G
I have updated my original post with the new info.