I have the same thing and the others I have seen are doing the same thing...hover over thumbnail and you see the exact same size as what is pictured then when you clik the image you get the ffull size (as to what you set--mine is 700x700)

I have different sizes for the individual settings but still only get the hover image to be exactly as the one pictured not the medium size.

Not sure if this is the intention of the add on or not.