If your page sizes are slightly larger than what you anticipated, it’s likely because you have an image set to bleed somewhere in your manuscript.
Bleed means that an image or illustration fully extends to the edge of the page. In order to ensure that the image always runs to the edge of the page, images should be extended beyond the final trim size. As a result, the exported PDF must also account for this by adding approximately .125 in to the width and .25 in to the height of each page.
If you don't wish to use bleed, you'll need to identify any images in your manuscript that stretch to the edges of the page and set them to appear within the margins only.