Images of equal width generate PDF pages of unequal widths

I made some webpage comps in Photoshop. Each page shares an identical width. Some pages were even made in the same Photoshop document, on multiple art boards (which all share the same width). The images were exported using Photoshops "Artboards to files..." export option. When I view the photos in Apple's preview application, you can see that they have exported successfully, at the expected width.

However, when I combine the images in Acrobats PDF builder, some pages are much smaller or much bigger than others. Has any experienced an issue like this? Is there a way to force Acrobat to match all page widths?

Thanks in advance!