Pretty sure I should be sleeping, but I had an idea to test. About 50 exports later I have things worked out…
It seems that in Windows, PNG inside PDF are saved as Indexed color. That is, 8 bit PNG. On Mac in PDF it’s saved as 24 bit PNG, and it’s the same when saved as PNG from either platform.
On Windows, if you set the resolution to High and ask for JPEG, you get what you asked for. So, in my test if I had high for resolution and highest JPEG compression, the PDF was 11 times as big as the same export without JPEG compression, because PNG makes the most of the flat colored areas in the model.
On Mac, if you have resolution set to High, you’re going to get 24 bit PNG no matter what JPEG compression you set.
Some of that may be intentional on our part or Apple’s part. But, it gives you a solution, if you set resolution to medium, the exports from Mac and Windows are almost exactly the same when using medium JPEG compression. Higher JPEG quality can make Windows seem to do better on file size, but the image isn’t as good as Mac with the same settings.
As an aside, if I take a bigger Mac PNG and send it to tinypng.com, it comes out smaller than the PNG inside a Windows PDF. So, there is a feature request in there somewhere.