combining files and page resizing.

I'm using Acrobat Pro XI in Win 10.  I started with double pages in one PDF file.  As I cropped them into single pages I saved them in separate files.  Then I combined them into one file, figuring I would put the pages in their proper order in the thumbnail column.  Everything looked fine in the thumbnail column but in the editing column some of the pages were much smaller than the others.  I fooled around in the crop mode but nothing stayed the way it should.  When I enlarged the page to 8.5"x11" the text stayed the same smaller size.  I closed the file and reopened it later and all the pages were aligned in the same size, with the text filling the page they way it was originally.  Don't know why but I'll take it.  So I attached the file to an email and sent it.  The recipient said the pages were all different sizes.  I opened the file again and there was the same problem; pages of different sizes. 

I'm out of solutions.   Anyone know how to fix this problem?