The forced perspective illusion in Main Street, as an example, has the second floor windows and facades be smaller than normal, giving an illusion of height. The castle does the same thing with each 'floor' being smaller and smaller.
It seems to me that WDI wanted people to look at SM from the point of view of a small critter, which is why the thorns are super-sized. So, that's a trick that works. Although... people's experience with thorn bushes vary. A thorn is not a regular measure of size that we use to gauge the relative size of everything else.
However, I'm of the opinion that the tree trunk ruins the illusion. It's there because it's there in the movie. But, it seems to make the drop small because the drop is not that much bigger than a tree stump. A tree stump is at most human height. But the one atop SM was much bigger. Which makes the drop look smaller, since the eye is seeing a drop that's barely bigger than a tree stump.
From my perspective, I've never seen the drop look amazingly bigger than it is due to forced perspective.
