In essence this relates to weapon damage from creatures using "identical" weapons: A human's long sword does 1d8 damage, sure, but a storm giant with a proportionally sized long sword shouldn't do merely 1d8 damage plus its strength bonus simply because it's described/shaped like a long sword.
So a pixie is Tiny while a human is Medium and a dragon is Gargantuan in size. In a scenario where you have all three lined up at a target range and they're all, say, 10th level casters, would the size and effect of their fireballs be identical in scope?
A pixie is a little, itty bitty creature. For it to cast fireball and for it to equal the size and destructive ability of a human caster's fireball would seem overpowered.
Likewise a dragon is a mammoth creature so if it cast fireball you'd expect the magic it channeled to be much greater than a standard human caster's fireball.