Importing images at different scales

First of all, there's no need to change width and height. Just uncheck resample and set the new value. No pixels are changed or need to change - it's just metadata.

Second, in explanation to the first: The real underlying issue here isn't the resolution as such. It's that you are Placing as Smart Object. Smart objects are a special construction with special rules.

If you just went with a normal copy/paste, instead of making smart objects - as I would - then it's just the pixels, and the ppi number doesn't even come into it. This is the true, native behavior of Photoshop and pixel images.

Smart objects were designed to fulfil certain criteria. One of the most important was compatibility with vector applications like Illustrator. Illustrator doesn't know what a pixel is. So then they had to find a way to make Photoshop behave in the same way as Illustrator/vector apps.

The only practical way to do that is to use the ppi number. This translates from pixels, that Photoshop understands, to physical measures, that Illustrator understands. So now they had a common language and could talk to each other.

The net effect of this is that a Photoshop smart object honors the physical measurement, not the pixel size. And the physical size is a direct consequence of the ppi number. Pixels per inch!  This is all a bit counter-intuitive to long time Photoshop users, but we just had to accept it and adapt to it. Those are the rules.

It follows from this that if you make sure all ppi numbers are the same - doesn't matter what the number is, as long as they're the same - then this translation is 1:1, and everything retains the same pixel ratio. If one number is different, however, then it will scale accordingly.

Bottom line - if this confuses you, just watch the ppi numbers and forget the rest. Or don't use smart objects. Just copy/paste.