The 737 Max is like double the size of the original 737 - roughly twice the passenger seats, more than twice the weight, roughly 1.5 times the length... How much larger can a plane get, while still keeping the same certification?
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)