The page sizes are looking different in your PDF because the images were originally set to different DPI (even if images are identical HxW in pixels). The good news is - it's only a display issue - and can be fixed easily.
An image with a higher DPI value would display smaller in a PDF (displays at the 'print-size' of the image). To avoid this, open each image in an image editor like GIMP or Photoshop. Open relevant image print control dialog box and set a suitable uniform DPI info for all the images. Remake the PDF with these new images. If in the new PDF images are too big - redo the DPI setting for each to a higher value. If in the new PDF pages are too small to read on-screen without zooming, again - redo DPI adjustment, this time put a lower DPI value. Ideally, 150 DPI should be good enough for images of 2500X2500 pixel - on a 17 inch monitor set to 1366x768 resolution.
BTW, the PDF file shall print each page at the specified DPI of that page. If all images are same DPI, you'll get a uniform printing.
Hope this helps :)