Notes? Are you using the TDP backup agent, or just the regular filesystem
client(dsmc)?

I assume dsmc.

So, backups happened...
then migration from DPOOL to TPOOL...
And now you're doing a "COPY STGPOOL TPOOL COPYPOOL" and seeing small files.

Well, MAXSIZE=5G means that during BACKUP, files over 5GB go direct to tape.
Maybe that happened OK.

Then migration put ALL the files, large and small, to TPOOL...
... and now COPY STGPOOL shows you some small files being sent.  Looks
normal.

What you might do differently is
   First: COPY STGPOOL DPOOL COPYPOOL
   Then:  COPY STGPOOL TPOOL COPYPOOL

The second COPY STGPOOL will not bother to copy files that the first one
found on disk - i.e. TSM understands that DPOOL -> TPOOL is a hierarchy and
can be treated as one pool.

(In fact if you have a big enough diskpool to catch all your incremental
backups, the second command will do nothing.)