I have a PDF (all images) which I have added pages to, two in the beginning and two at the end. The added page are displayed about one third smaller than the original pages, even though they are all in dimension size. In fact most of the original page aren't the same dimensions either, but they are all displayed the same. The originals DPI is 96 while the new ones are 200, I don't think that matters though.
A quick example:
New page dimensions;
Original page dimensions;
I have a very similar problem. I have a 26- page pdf, all images, all but two (first and last) are square. I resized all images in MS Paint to either be 1000x1000 if square, or 1000 wide if not. The second image appears at less than half size, and images 4 and 5 appear at about 90% size, despite having the same dimensions as the other pics. Before resizing, these images ranged from 650x650 to 1200x1200.
I can scale up the images in Edit mode, but the pages themselves will not scale, which acts to crop out all image area not contained within the original borders, and attempting to scale the pages gives me a "Page sized may not be reduced." error, despite me attempting to increase the page's size.
The best I can guess is that Acrobat sizes images based on pixel density and not actual dimensions. But if that is the case, I need to know how to disable it, if possible.
The size in pixels is not relevant to a PDF. Only the size in inches/mm. You should fix this by setting a consistent resolution before making the PDF. You'll need something more powerful than Paint to do that, I suspect.