copying layer from one image to another

I have a number of 5X3.5 images and I want to copy 2 of these into a
new 5X7 file I've created.

Currently I am just dragging and dropping over the layer from the
smaller photo and placing it in the larger file.

I'd say 60% or 70% of the time the image that I copy comes over much
larger than it is in the original. Is there some setting I'm
accidentally turning off or on that is causing the image to copy over
at a different size than it actually is? How do I get the image to
stay at it's current size?

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