In this kind of scenario, I expect the block size to have only a marginal impact. Indeed if all the CD ISOs are very similar, I would expect that the size of a duplicated chunk to be on average quite big. The difference between using 128k and 64k for BTRFS is for instance not very big.

But except for the block size, I don't see other explanation for the differences.

Dedup is dedup, so I failed to understand why results between different implementation should lead to such differences at the end (except very incorrect implementation !).