A lot of your comments are open to interpretation so it's not easy to establish exactly what your workflow is.
For example:
So this must start out as a normal image file, right? You then, for some reason, "resize" it.
Now, putting 'why' aside for a moment.....HOW are you resizing it?
Do you mean you resample the image so it contains less pixels?
Do you mean you use the 'resolution' setting without resampling? (Which is just calculating a smaller print size)
Are you selecting everything and simply transforming the image smaller?
In short, 'resize' could be interpreted in different ways, and HOW you are doing this can dictate what happens to the file afterwards.
To continue...what do you THEN do with the file once you 'resize' it?
Save it as the original image file? (Overwriting in the process)
Save it as a new file?
Save it as a PSD file?
None of the above?
Then there is the 'dragging' part....WHAT are you dragging? From where? Are you dragging the previously saved file? Are you dragging the transformed pixels from one document to another?
You see how there are many, many, different permutations....and just in that one sentence alone!
Without expanding on your exact workflow it would be nearly impossible to determine exactly what is happening and why.
Normally, dragging layers from document to document isn't affected by size, resolution setting or any document specific properties....the pixels just get copied over to the target document.
Problems can arise when dragging [saved] FILES into documents, as
colleague has already mentioned the resolution setting will make a difference.
Personally I also don't understand why you are 'resizing' the file first before transferring to the target document. Normal practice is to 'Place' or open and then drag the file directly into the target document and then adjust it there, not before. Is there a reason why you cannot do this?
If you do, does it make any difference?
Dragging pixels from doc to doc is a direct 1:1 pixel relationship so I'm at a loss as to what is happening to them in the process.
Regards.
MrToM.