The red squares are exactly the same size. However red appears more vibrant against a black background and duller against white. In contrast with orange, red can appear lifeless. With blue it appears brilliant. So the red squares can seem as though they are different sizes.
The same colour can appear to be different depending on the relationship and size of adjacent colours. Which of these pairs of complimentary colours do you think has more impact? For example, the red as a shape or the red as the background?
If the sizes of the squares and the ovals were different, you might have a different answer! Adding the same colour to different backgrounds can make it appear to be two different colours.
The circle appears to take on the cooler tone of the blue background, or the warmer tone of the aubergine background.
How a colour appears to behave in relation to another colour is a complex area of colour theory. We don't need to understand all the scientific complexities, but we do need to be aware of this conundrum and experiment with the way we use the colours we choose.