I question this as well. What people don't realize, is that, in Butterfly Mode, Chōji's defense is just unreal. In order to even deal any type of significant damage on Chōji; one would first have to breach his thick layer of hard skin that can withstand continuously tearing through the earth and the Gedo Mazo attacks from point blank range. Keep in mind everything has to grow in proportion to his size, so if we take into consideration that the top layer of human skin is anywhere from 0.5-4 millimeters thick; and the fact that Chōji in BM is like x100 (just a guesstimate) human size; we can conclude that just the top layer of his skin is roughly like 1-2 ft thick. If you're lucky enough to breach his tough skin, then you'd then be facing a wall of fat/muscle which plays a huge part in ones durability [
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]-[
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]. And I'm not even getting into his other defensive capabilites such as chakra shrouding his skin for added defense/offense. All in all, Mei is going to have a hell of a time putting Chōji down. Her acid mist isn't getting past his skin before it get's dispersed by his huge wings or a rollout, suiton isn't accomplishing anything at all, and her slow lava can be avoided or simply withstood like so [
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].
I don't believe I have to mention his offensive capabilities simply because it should be common sense that Mei get's flattened easily. But I will if I must.