You guys are all rocket scientists. [:-]
Here is the explanation, from a high school drop outs perspective.
First off. the funtana is blurred because he said he was following the P-47 with the camera. So the P-47 is nice and clear. The trees in the background are blurred because the camera is moving with the P-47, then the funtana is moving in the opposite direction, which makes it appear to be twice as blurred than the trees. If we knew the shutter speed of the picture, we could then estimate the speed of both planes by measuring the blur of the trees, and the funtana, if the resolution was better.

The funtana appears to be larger because its wings are at more of a tilt, and due to the lack of depth perception, as someone stated, it makes the funtana appear like it has the same wingspan as the P-47.
The P-47 is 1.125" long in the photo, and the funtana is 0.9375" long.
Looking up the fuselage lengths of both planes we come up with 70" for the P-47. and 56" for the funtana .40
We divide 70" by 1.125 and get 62.222.
If we take the 62.222 number we can then get a relative number of length for the funtana 56" / 62.222 = .875
Which is only 0.0625" or 1/16" shorter than it would be if the 2 planes were occupying the same vertical plane.
At 50-100 feet out from where the picture was taken, 1/16" is not very much difference, and that would put the 2 planes pretty much over lapping each other.

If the distance from the camera was 75 feet. Using an angular distance calculator, and a difference of 1/16" apparent size, I came up with 1" separtion on the vertical plane. So yes the funtana was on top of the P-47.

This is all an estimate, and you must take into account the angle of the 2 planes relative to the direction they were flying, that affects the measured size I got from the picture, and the resolution of the picture. I was measuring blurry pixels.

Another note, the P-47 looks super imposed on the picture due to the way the sun is hitting both planes. My guess is the sun was at about a 45 degree angle and up to the right. So the fuse of the funtana is dark because it has no curve to it, and the P-47 has more of a curve to hit, so the sun appears lower. Plus the tilt of the wings on the funtana makes it seem worse.

Great picture, you should be proud!