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When you're working with multiple characters, it's important to create more than just a unifying visual theme, but differences as well. You don't want them to blend together to such an extent that they lose their individual personalities.
One way to do this is to vary the height of the characters. Varying the heights forces the eye to travel up and down while scanning the image, which makes the eye slow down and take more time to soak in the image. When the charactesr are all the same height, the eye tends to skim the image quickly, as one whole clump, without looking at each character individually, which is a disadvantage for the artist, who has worked hard to draw each figure.