If you check the bleed in the options section of the Export as PDF dialog, you will probably find that it is set to add some amount of bleed to the size of the PDF. Try setting the bleed type to Page Items, or if you want to leave it as symetric or asymetric, set the amount to zero. The Marks section of the options will also add size to the PDF when it adds marks, so leave them out if you don't need them.