I agree the pic looks to be a manip - biggest deal for me there is the area near the fox's head and forelimbs - at 16X the pic on the other forum, you plainly can see unnatural blurring, pixelation appears different from the pixelation of the surrounding image, and, biggest deal, just to the left of the fox's right leg, there's a distinct 90ยบ vertical line - sure looks like an paste-in artifact to me. However, the scale comments don't really move me much - to my eye, the stuff there does look reasonably proportionate, especially considering the eagle, the fox, and the magpie. The presence of a magpie in a real scene of that sort is very possible, BTW - magpies like carrion, too, and its fairly reasonable to expect a fox might not put up much fuss over sharing a large carcass with a magpie - I've seen fox sharing roadkill with crows and magpies before. Eagles, however, ain't much into sharing with anything other than that eagles mate or chicks. Still, I lean toward thinking the photo is fake - even if the premise is plausible.
Here's one for ya - not "shopped" - a juvenile Baldy (prolly its 2nd winter - they don't develop the white head and bright yellow beak 'till around 4 to 5 years of age) enjoying a red fox: