Why is there extra space in the canvas of my images after loading files into a stack?

What does one of the JPEG images look like when you open just that one in Photoshop, without stacking it with the others? Here is why I ask:

When stacked, there is a lot of transparent area along the right of the canvas. But…JPEG does not support transparency, so it shouldn’t be something in any of the JPEG files. If it was, that space would be solid white, not transparent. That is what drew me to the one with gif in the filename, it became the prime suspect because if that is actually a GIF file, of a different size, it would have the ability to have a transparent area that extends.

Another troubleshooting approach to try:

1. Select the Move tool, then in the options bar, enable Show Transform Controls. This will display the bounding box to visually indicate the full dimensions of every layer. 

2. Select each layer one by one in the Layers panel. Watch the handles.

My guess is that at least one layer will show transform handles extending over the transparent space on the right. Any layers where the handles stop at the right edge of the photo stack are innocent. Any layers where the handles extend all the way over the transparency are causing the unwanted extension, and should be inspected further.

Also, it’s possible that an auto-alignment error has positioned at least one layer outside the canvas; the visible transform bounding box should reveal that too. But so would Image > Reveal All.