I'm not sure what you mean by this. When you open an image in Photoshop, it will change the zoom if at 100% it is larger than the available display space. Often it will round this off to certain percentage that corresponds with a simple fraction, like 2/3 (66.66%) or 1/5 (20%) which may be a little smaller than the available space and appear smaller.
If this isn't the case, please post a screenshot of what you see.
Check your ppi values you've set in the metadata for the respective images (e.g., with Image - Image Size, and the "resample" option unchecked). I think you'll find they're different.
There is a "Resample image during Paste/Place" option in the General options that you may want to uncheck if all you want is a pixel for pixel operation.
That screen shot says a lot.
First off, you are zoomed out to 3%. This document that we are looking out is ENORMOUS!
The file that you are placing is currently centered in that document. Its coordinates are 21199.5x and 11920.5y, which means the entire dimensions of your document are 42399x23841. If you place a file that is only 2120x1192 within that, it will take up about 5% of the available space.
In other words, what we see is the correct behavior, you are placing an average sized file onto a very large canvas.
When you create a blank file, make sure it's closer to the size you want to work with.