It's the DPI--totally. When you copy and paste, it will change the DPI to 300 for you. When you drag it over, it keeps the DPI, so it gets resized.
An easy fix: resize the document... But only change the DPI. Keep the size (1.5) the same, but change the DPI to 300. It won't really resize it, but it will now be 300 and you won't need worry about it again.
