The final product of a (La)TeX run is something for a person to read. Often, nowadays, that product will be read “on-screen”, but the printed page remains a principal output form.
When we come to print our output, it is important that the output fits on the paper; in some cases, for the output to “fit” is good enough. However, there are circumstances where the actual size of the printed output, on the page, is crucial to the acceptance of the output. (This might happen when the output is a book to be published, or when it's a dissertation which must match the fancies of some bureaucrat even to be considered.)
Sadly, we often find that the printed output doesn't conform to our expectations…
The check-list for such problems has two entries:
Acrobat Reader- older versions of the program had the qualified name). In this case, it may be that
Readeris willfully changing the size of your output: read `Reader` antics.
An alternative approach is to use the (excellent) testflow suite, that provides a detailed outline of the potential problems together with a sample document and prototype outputs.