Was dealing with a big Powerpoint presentation (PPT) file.
In the older PPT format, 6063 Kb.
When zipped, 4826 Kb. Not a bad saving given the number of pictures in it.
Here’s the interesting thing: in PPTX format: 3293 Kb.
Remembering that PPTX and other Office Open XML formats (DOCX, XLSX etc) do their compression on the file as a whole, not the individual componenets, so this is an interesting result.
Perhaps the old binary format is inherently less efficient/compressible than the new XML format.
Mind you, another big PPT I tried it with didn’t compress down as much; the PPTX was about the same size as the ZIPped PPT, so it obviously depends on the exact content