Interesting question! First, let's make one fact clear: in general, color does not affect size. You can easily draw a black square that is much bigger than a white square, or a black square that is much smaller than a white square. If you tell the computer to make two squares that are both four inches tall and four inches wide, they will be the same size no matter what colors you pick.

But color can affect your perception of size. Even if you know that two shapes have the same size, you might feel like they have different sizes because of their colors. This is an example of an optical illusion. An optical illusion is an image that tricks your mind into seeing something different than reality. The branch of science that studies how our minds work is called psychology. The scientists that study how the mind works are psychologists.

Here are some famous examples of optical illusions:

In the simultaneous contrast illusion , the changing background makes it seem like the grey bar goes from light to dark (from left to right). But if you cover up the background (try holding some paper up to the computer screen), you will be able to tell that the bar is the same shade of grey all the way across.

To see the revolving circles illusion , look at the black dot and then move away from your computer screen or towards the screen. Nothing in the image is moving, but you will feel like the circles are rotating when you move.

I would say that the lesson to learn from optical illusions is that you can't always trust your senses. It's good to double check things that you see (or hear) by another method. For example, you can measure the shapes in your presentation with a ruler. That way, your perception won't be affected by the color, so you'll have a more accurate idea of the size.

There is one more possibility for why white shapes might appear smaller than black shapes in your presentation. In some computer programs for making presentations, shapes appear with black borders around them. Since the border is black, this border will merge with the black shape, but not with the white shape. Try changing the border to a different color like blue for both shapes, and you might find that they appear to be the same size.