Safari is scaling down large images for what should be fairly obvious: it has only 1GB of RAM. An 80 Megapixel image, regardless of whether it's a PNG, GIF or JPEG, decodes to the same size: 240MB (3 bytes per pixel, one each for the red, green, and blue channels).
That's almost a full 1/4 of all of the RAM on the iPad, for one picture! Of course, photo apps that don't downscale the image will necessarily use that much RAM to view it, but this can be allowed since you can generally trust the images already on your device.
Safari has to contend with image bombs. For example, using pixelmator, it is possible to create an 18250 x 18250 white (or other solid colored) image, which compresses down to a 1MB PNG file, yet decodes to nearly 1GB. If mobile safari naively decoded this image at it's full resolution from a malicious website, Safari, and possibly all of iOS would crash in a rather predictable manner, which may be able to be exploited to run arbitrary code.
In other words, down scaling images is a security measure! Now, Safari could be written to downscale the images less extremely on the iPad 3 (probably 5 Megapixel images would be okay), but it has to be done.