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>Or did they do it to make him easier to animate.
This is the actual reason, same reason starters have an almost identical body type these days. Its easier to make merch and animations of tiny humanoids than it is to make the same thing for a really fat mouse. I'm also pretty sure that fat Pikachu is just one cap from the anime pilot, even in the OG anime, Pikachu was pretty slender and was really only super chunky in the first episode.
