aperture - Why is the background bigger and blurrier in one of these images?

On my EM-10 Mk2, I'm testing a Panasonic 25mm f1.8 (top image) against the 20mm f1.7 (bottom image).

I tried to make the lamp take up the same space in the frame before focusing, yet the background in the case of the 25mm is much larger and blurrier than the 20mm, which I actually prefer.

Since the aperture is nearly the same size between the lenses, is this simply a function of focal distance? Any explanation about this would be very much appreciated!

Answer

In the case of the second shot (20mm focal length), the camera was positioned closer to the lamp, thus making it appear bigger than the background.

As the camera moves further away from the subject, the distance between the subject and the background becomes smaller relative to the distance between the camera and the background, making the subject appear more and more closer to the background.