If you're just changing the ppi resolution without resampling, the pixel count remains the same.
If you were to print your file at 140 ppi, the print dimensions in inches would be approximately 14" larger than at 160 ppi.
Also, it's entirely possible for a an image to be compressed a bit differently each time it's saved, as the calculations are made from scratch.
Nevertheless, I agree that your results seem pretty weird.
Try setting the "Maximize Compatiblity" setting to "Never" in your File Handling preferences. That might save enough space and also allow you to save the file more quickly.
I understand re-sampling and I was definitely doing that. Also, I don't believe 'maximize compatibility' is available for .psb files. I definitely wasn't imagining things and even had a couple colleagues check the files as well. Very odd. It would be nice to solve this mystery for future reference as it is pretty hard to find a service that transmits files bigger than 10 gigs and it takes over an hour to save these files.
… don't believe 'maximize compatibility' is available for .psb files…
It was Noel who suggested that, but I can confirm you are correct. I just tried.
I didn't know whether the compatibility settings applied to PSB or not. Thanks for checking and setting me straight.
I appreciate all the input. If I have time I will try some experiments to see what parameters this occurs under. Seeing that it only seems to take place with large files that take nearly an hour to save, I may not have that kind of time in the near future. I guess the lesson to be learned is if you are rushing to deliver a huge file to a client, you may be better off putting it on a thumb drive and taking it to fed ex rather than expecting your ppi reduction to make it manageable every time. I did finally get it to a manageable size at 100 ppi when it dropped to 9 gigs. I was sweating that one out because I was watching it save as the Fedex deadline passed.