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You're generally right that a 700c with the same size tyres is more in proportion, but it'll be heavier and not as strong as the 650B counterpart, plus, the rear wheel wouldn't be "slammed" with a 700c of the same tyres size.

The chainstays would have to be longer with 700C, which would actually be an unambiguously good thing

700C would be heavier, especially after adding some meat to keep the strength equal, but the lower rolling resistance would completely overwhelm the small weight penalty, making the bike faster, especially over rough terrain.