help automatically resize from different files

We can not see document dimensions what is important. Can you take screenshots of entire window and post here? Your documents appear at the same dimensions visually but that does not mean they are identical since documents can be displayed at different scaling factors.

Your movie is difficult to see. Your iMac comes with two applications that can record video:

• Screenshot
• QuickTime Player

https://support.apple.com/en-us/HT208721

I agree with Dag. Set the unit of measurement to pixels in both documents and you will see that both images retain the same number of pixels.

Jane

Have you tried placing it as a smart object rather than copying/pasting it?

An image is just pixels. When you copy/paste, the pixels align to the pixel grid in the base document 1:1.

If you paste a 200 pixels wide image onto a 1000 pixels wide image, it will fill 1/5 of the width.