Don't confuse it with 'dimensions' (width and height). The image's resolution relates the pixel dimensions to real-world measurements. When you drag in (or Place) an image, Photoshop uses the resolution to determine how large the image will initially be so their real-world dimensions will be consistent.
You can still resize the image by dragging its corner handles—it is most likely pasted in as a Smart Object, and can be fine-tuned until it is rasterized. If you need the images to match pixel-for-pixel, you will need to change the other image's resolution in a separate document.
Answers & Comments
Verified answer
Resolution.
Don't confuse it with 'dimensions' (width and height). The image's resolution relates the pixel dimensions to real-world measurements. When you drag in (or Place) an image, Photoshop uses the resolution to determine how large the image will initially be so their real-world dimensions will be consistent.
You can still resize the image by dragging its corner handles—it is most likely pasted in as a Smart Object, and can be fine-tuned until it is rasterized. If you need the images to match pixel-for-pixel, you will need to change the other image's resolution in a separate document.