JPEG compression is all over the map. Going for a file size in MB with a JPEG is totally nonsensical. Try the quality settings and you'll see for quality 10, 11, 12 you may see file sizes of 3 MB, 6 MB, and 18 MB (or something), with no visual differences you can see.
Uncompressed (TIFF) is different: There's an exact file size for a given megapixel size (and depending on whether it's 16 bits or 8 bits per pixel).
Exactly. File compression works by throwing away redundant data. JPEG compression changes an image to increase redundancy, whereas PNG's compression does not change the image, it only attempts to find what is redundant and describe where that redundancy is. Either way, a compression algorithm's resulting file size will vary greatly depending on image content.
For example, the file size of a PNG for a computer-generated illustration is much smaller than a photo that has been compressed with PNG, since the computer-generated illustration has more areas of solid color, and thus more redundancy.
Aperture's flavor of TIFF is uncompressed, although other programs do offer various compression methods.
This site has an interesting comparison of various compressed image file formats: http://www.sitepoint.com/gif-png-jpg-which-one-to-use/