VSA replication - Different Size on disk between source and target

Dear All,

We are testing two Storeonce VSA systems with one vtl replicated on the other VSA vtl.
Could you explain us why we have differences in Size on disk between Source and Target?

For example we have a source size on disk (dedup) of 338GB and a target size on disk (debup) of 778GB.

Thanks in advanced,

Salvatore