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As has been pointed out, Raichu and Gyarados are the same size when Dynamaxed, so this shows that they don't grow relatively.
My theory is that Dynamax forces Pokémon to reach the same mass (perhaps the mass of a legendary?) and Gyarados was already nearer that mass than Raichu was so it seemed to grow less than Raichu did to reach that mass.
So by that logic, a Wailord wouldn't actually grow too much, whereas a Joltik should get MASSIVE!
