SWF decompres/compres -> different size

Hi,
I am trying to decompress a swf with Swifty Compress & Decompress.
But when I decompress and then compres it back (an unchanged file) it changes filesize.

So even without any changes in bytecode i get different file size :/.

Anyone know what should I use to decompres/compress without changes?
Swifty is pretty old, maybe it wants something newer..

Thanks in advance for any help.