1) You are probably looking at background images. Adblock Plus doesn't list the intrinsic size of images but rather the size of the element the image is associated with. As currently implemented in Gecko, background images are not associated with any particular element of the document but rather the document as a whole - so deducing size is not possible.
2) Tooltip image previews only work for images
An object subrequest can be anything and determining that it is an image isn't quite straightforward.
Thank you for these points, I think that fixing them in one way or other might be worth considering. Size of background images can be looked up in the cache (I think that Page Info dialog does it like that), same goes for the mime type of object subrequests to determine whether it is an image (if it is in the cache at all of course). And - yes, file sizes are available in the cache as well. The question is only whether cache access is fast enough to do this. I'll move this to the Future Development forum.