If that's the case it means the files didn't have the same page size in the first place. Acrobat does not modify the page sizes when combining files, not to make them identical and certainly not to make them different from one another...
Thanks, but they were the same size. Perhaps it is due to different originating systems. Or, because I used “Comments” on each page, then saved, then “Combine files into one pdf . . .” it made the page sizes different. Is there a way to know what Acrobat “sees” as the page size before performing the “combine” function?
Maybe they look the same, but are not. You need to look at the zoom level you're at, not just at the size of the page on the screen...
And adding comments has no influence on the page size.
To see the actual page size go to Edit - Preferences - Page Display and tick the box that says "Always show document page size".
Then it will appear at the bottom left corner of each page you open, for example: