My table already has an uncompressed binary field
So what I did was :-
1) Add a new binary column and tick 'Compressible' box.
2) copy the data from the uncompressed field to the new compressible field with
update att set COMPRESS = UNCOMPRESS
3) Remove the old uncompressed field.
4) Packed the table.
The file size of the ADM file is exactly the same.