Lying down, your
weight throws the ass downwards and the shoulders backwards as compared to a standing position ...
(angling the hips backwards and stretching the torso longer)
which is even worse on a soft surface.
As a bencHydromaxark, you will be further away from a "fixed" (but actually moving relative to the penis attachment point) item like the belly button. If someone is trying to measure you by that you'll seem smaller.
If there's a psychological trick for getting around this, it might be to set the image of your size for the other person in some other position before lying down. They'll be thinking "Hmmm, how the heck am I going to get all of that inside my x,y, or z", rather than pulling out a sliderule.
rofl
Try moving your hips forward -- like pressing your low-back into the surface -- and you'll recover most of that loss. (There'll still be some torso stretching from the shoulders pressing back).
Do a 'scrunch' sit-up with a hard-on and the mechanics of all this will be really obvious.