When I combine PDFs I get different page sizes in the new PDF. Why?

[Combine PDF pg size diff]

I used the Combine PDF feature today and found that the final version of two originals, combined, had different page sizes when viewed in Adobe. How do I fix this issue? Does it have to do with DPI resolution on scan?

[Test Screen Name]

This will happen if the PDFs you are combining have different page sizes. You can check this by hovering the mouse in the bottom left of the screen, at least in Acrobat.