Mmmm, I think there's some confusion here. The 737 MAX has its own certification, as has every generation of the 737 (Original, Classic, Next Generation and now

Mmmm, I think there's some confusion here. The 737 MAX has its own certification, as has every generation of the 737 (Original, Classic, Next Generation and now MAX). However, if a pilot already has another certification of the same type then they are only required to take a differences course to operate other variants (which comes out to a few simulator sessions and a couple of check rides), as opposed to earning an initial type rating from scratch (which can require over a month of instruction). Also it's entirely normal for a variant of a vehicle to have significant physical dimension differences. They're considered variants because their [chassis, airframes, hulls, etc] follow the same design and assembly, and because they handle in a similar way; they don't need to basically the same vehicle in a different coat of paint. I guess a somewhat decent comparison is...Apple makes a type of computer (the MacBook Pro) and releases different variants (and within that, versions) of it. A fully specced out 2023 16-inch MacBook Pro and its entry-level 13-inch variant are both fundamentally the same type of computer even though the former is significantly bigger and beefier. Now, if Apple tried to shove a whole RTX 4090 graphics card into a MacBook Pro's characteristic svelte and relatively vent-less chassis we would say "no, that's stupid. Make a different type of computer if you want to do that". But that doesn't and shouldn't mean that a 16-inch laptop is a different type of laptop from a 13-inch one. (As an aside, your numbers are a bit off, probably because you're comparing the wrong things. The "original 737" is two planes/variants, not one, and the "737 MAX" is similarly four planes not one)