Go to your page setup or printing window and uncheck or change the setting for "Shrink to fit" or "Fit to Page". The precise term is different in different versions of Acrobat and on different platforms.
What Acrobat is doing is saying, I know what your printer's limitations should be and I'll shrink all the information into that area.
This works well unless you need 'precise size or alignment'.
Hope that helps!