In DSpace 4 and 5 the versioning duplicates the item, all bundles and bitstreams whenever a new version of an item is created. While doing so it uses the same internal id of the duplicated bitstreams as in the old version. This is done to not use unnecessary much disk space as the internal id is the link to the file on the disk. With DSpace 6 this behavior changed and the file on the disk is copied. This produces a new internal id, but that also uses more disk space then necessary and takes more time while creating new versions (I can even imagine situation in which this could yield to a timeout).
We should rebuild the old behavior in DSpace 6 and reuse the same internal id of a bitstream that is created during versioning of items.
See also https://wiki.duraspace.org/display/DSDOC6x/Item+Level+Versioning#ItemLevelVersioning-Versioningmodel that describes how it should be.