Rauno: Here's why it's possible:
the first assumption that most people make is that all of the parts put together as they are make up a triangle, this is not true.
the smaller green triangle is 2 x 5 (40% slope)
the larger red triangle is shallower at 3 x 8 (37.5% slope)
on the top set, the green and red triangles create a concave(inward sloping) surface, and when switched they create a convex(outward sloping) surface giving you the size difference.
Now onto my own submissions to this thread
Commentator who goes under the name Koaieus left an intriguing image on one of our last posts. Even though I prefer illusion submissions sent in through ordinary email, I suppose it can be done this way too. Anyway, this one was really hard to see. Can you spot the hidden motive beneath the layering stripes? It sure took some time before I could see it. Be sure to un-focus your vision, and lean to the side while observing the given example. If you’re still having hard time seeing it, check out the title – perhaps it might help!
This one was awesome!
Found it on some illusion-website :P
EDIT, a few more, just for kicks
(Static image, believe it or not)
(Fake static 3D)