Some time in the past, I changed a PS setting that I can't now find. I changed the default resolution for imported (from my camera) or opened (from my desktop) photos to 180 ppi. I now want to change that to 300 ppi and can't find anyplace in the PS prefs to make that change. How do I do it?
Your issue is very simple, ido, and understanding what happens may help you avoid confusion in the future.
Smart objects honor physical print sizes, regardless of pixel dimensions or ppi. It's done this way to make a pixel image mimic the behavior of a vector object, which is the only way to get compatibility with Illustrator.
But since print size is a direct result of the ppi number (pixels per inch!), a 300 ppi image placed in a 72 ppi file gets smaller. It's scaled/transformed.
If you simply copy/paste, instead of Place, you're dealing directly with pixels and this doesn't happen. It's only smart objects that behave in this non-intuitive way.