Combining files to a single PDF does not cause them to have the same page size. The original page sizes are maintained, even in the new file. That is by design.
If you want to make sure that all pages have the same size you should do so before converting them to PDF. Alternatively, you can scale the PDF pages. If you have Acrobat Pro DC you can do it using the Preflight tool. If you have any other version you can use a plugin or a script, like this one I've developed: Custom-made Adobe Scripts: Acrobat -- Scale PDF Pages
I have a similar issue when combining Word and Excel 2016 files into a single pdf , when scrolling the Excel file appears a different size however it prints OK. Any help much appreciated
Try setting your view magnification to Zoom to Fit Page (control/command-zero).
Or press this button:
The idea is that pages in a single PDF file can be of different dimensions, but if viewed to fit the page to the view panel, they will all fill the space and appear similar (regardless of their actual dimensions). This is a great strength of PDF file, IMHO. If this helps, and you want other to see the PDF the same way, you can visit the Document Properties (File>Properties), click on the Initial View tab, and set the magnification to Fit Page.
I hope this is helpful.