Doc, I've just done a test with an image I had uploaded to my FB wall and I'm wondering if while the images are displaying on FB, it is an optical (FB) illusion that they are enlarged? Because when I right-clicked the image and opened it in a new browser page, it was enormous.
So I then saved it to my desktop and checked out the properties (and also in an image re-sizing program) and the image is exactly the same size as it was when it was snapped by my phone.
Can you try this with one of your photos on FB and see if you get the same results?