While working I’ll more than often just place documents as smart objects. Then I’ll end up with a colossal file size. I want to go into the smart objects and reduce its image size but consequently the parent psd will change the layer size of that smart object. I also don’t want to resterize those layers.
If you change the size of a Smart Object (embedded or linked) by changing the Resolution the size in the parent document should be unaltered.
If you on the other hand change the size by changing the Width or Height it will also change the size in the parent document.
(The same principle applies to images placed in InDesign by the way.)