I kind of worked it out.
This is a rough approximation of where the shadow is, with an estimation of Bowser on the left. A little got cropped off on the right side, and Bowser is obviously going to be smaller due to perspective. But not much.
The blobs are kind of too close for a boss at all. Like, maybe it's just me, but I am pretty sure if Ridley was a boss, there would be absolutely no mistaking it when comparing his size to other characters. And I mean if anything, his shadow would probably be larger because of his wings. So I have to wonder.