File size difference between PF and *SAVF

This may be old stuff. Just noticed today. I had to transfer a large PF
from remote 520 to local. Learned, in the past, that FTP does not retain
file definition, so created a save file and transferred it. Paranoia about
file content led to extracting the single file and spot checking.

Never had the opportunity to do this before. I thought a save file might
be a little smaller than original physical file. Maybe somewhere, but not
in this case.

Save file size is 18586152960
Physical file size is 12014727168

I expected the save file to be a little larger, to acomodate field
information. I wasn't aware the difference could be this much. I did not
specify compression on the v5r3 system.

Does the size difference seem unusual, or is that the norm?

John McKee