What Creates This Optical Illusion?

It isn't an illusion. It's augmented reality.

There is very little information about the items in your example. So the brain seeks additional information through context.

Let me illustrate. Take the question:

"How important is tweadlfark?"

Your first question is "wtf is tweadlfark?"

Your second question is "how can I found out more so I can put tweadlfark into context?"

Now your brain has a clever mechanism that seeks out differences and enhances them. That's because differences matter in nature. The difference may be that there is only one fruit that is ripe or, more importantly, one of those mushrooms is poisonous. So the ability to see differences is an essential survival skill. So, surprise, surprise, your brain emphasizes differences. This is what's happened here.

In the one on the left, the orange circle is smaller than the others so its smallness is emphasised. On the right, it's the largest so it's largeness is emphasized.

However, I lied.

There is an illusion here. It's the illusion that there is only one "correct" answer as defined by the arbitrary idea that they are the "same size" or, should I say, that "same size" is the defining characteristic of this comparision and all other considerations are null and void and should take no part in the discussion.

The logic is that because they're the same size, they are the same in every other respect. When said like that, you can see how silly the idea is.

Take this picture:

Here, the man is larger than the Eiffel Tower. So is that an illusion? Well, it may be. That could be a model of the Eiffel Tower built to fool us. But the con is all in the 2 dimensional distortion of reality that is the photo.

It's like asking why a 2 dimensional representation of a 3 dimensional cube looks like this:

It is the distrorted attempt to suggest 3 dimensions when you only have 2 to play with.

But which one is right? They're all right in that that is how a cube can look from a particular angle but they're all wrong if they claim that that is the only way a cube can look from any direction or that one direction is more important than another.

And that is the illusion.

This is a clear case of mistaking the map for the territory.

Let's take a more famous example

The words underneath say "This is not a pipe".

And they're right, it's not a pipe. It's just blocks of various colours on a screen. We resolve those colours into a definable shape and then determine the 3 dimensional object that best "fits" that shape and decide that it is a pipe and, in fact, is a a particular pipe that is usually associated with Sherlock Holmes.

But, of course, it's only a pipe if we recognise it as a pipe.

A plumber would say, "That's not a pipe. This is a pipe"

And they would be right

A musician would say, "That's not a pipe. This is a pipe"

And they would be right

I see this kind of stunted thinking all of the time in lateral thinking puzzles - which is ironic when you think about it.

Let's take a classic example:

There are three objects in the damp grass under the bright sunshine. What are they?

The usual answer is two pieces of coal and a carrot which were the eyes and nose of a recently melted snowman as demonstrated by the damp grass.

The real answer is: they can be anything you can think of. For example, they might be a rock, a goldfish and an overturned goldfish bowl where the aforementioned rock hit the bowl spilling the contents onto the now damp grass and leaving our poor goldfish to its fate under the sunshone. Or it could be a gun and two gloves thrown out of the window of a passing train after a murder wher the grass is damp with morning due and has no immediate relevance to the identification of the objects only the time they were placed. Or it could be an alien and their robot which has recently parked their alien craft (the third object) there for a picnic only to be disappointed because the grass is wet from recent rainfall. The number of "correct" answers is near infinite.

The illusion is that when you limit the amount of usable information, you also limit the number of "correct" answers to that information when the reverse is actually the case. As, less information is given, more has to be inferred to construct a sensible scenario and in that inference, the number of options multiplies exponentially.

The illusion is that the assumptions we make are the only assumptions you can make and all others are wrong.

In case, you're wondering a tweadlfark is a type of tuning fork used to find the note of the human aura so you can re-tune it to be in harmony with the music of the spheres.