Perspective and angles have a dramatic impact on perception. I really think you should work with some simple white props until you have a firm grasp of what to expect. Also - this has the benefit of producing a render in 10 seconds or less which permits vastly more effective experimentation.

Here is a scene I set up with a gel similar to yours. Two renders with different camera focal lengths and perspective (position) reveal a big change in the apparent size of foreground vs. background shadows. If this isn't clear to you, then do 100 renders like this - your brain will learn what to expect without you thinking about it.

Here it is with the camera focal length at 26 mm

And here with the camera focal length at 91 mm

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)