autoresize in Safari actually resize the image data

[balmydrizzle]

I use Safari on my iPad to connect to a very big jpeg file (e.g. 2000x3000) directly via an url, say 'http://mywebsite.com/A01.jpg. I thought Safari only autoresize it on screen but still keep the original image file. So when I enlarge it, I except it'll be clear image due it's a very big image. Instead, I found it's very vague. Does Safari actually downsize the image data?

I want Safari show original image data when enlarge the image on screen, how can I turn off the autoresize?