For example the Nikon Z 50mm f1.2 is 1090 grams, 150mm long, and has a 82mm filter size. The Canon RF 50mm f1.2 is 108mm long, but the other dimensions are similar.
Compare that to a Leica Noctilux 50mm f1.2 with a Techart, Megadap or similar adapter (available for Z and E mounts) for autofocus abilities: 405g lens +150g adapter = 655 grams, 52mm lens + ~11mm adapter = 63mm long and 49mm filter size. A little more than half the numbers in all dimensions.
This link approximately shows the size differece (the M to L mount is indeed smaller than the M to Z or M to E autofocus adapters, but the difference is small)
All of these have the same focal length (50mm), max aperture (1.2), and autofocus. So why do these newer mirrorless lens designs have to be so much bigger and heavier than using an old manual lens with an autofocus adapter? Sure the autofocus speed may not be as fast with an adapter but why can’t they design a native autofocus large aperture lens that is tiny like the Leica M lenses. Clearly it is possible to do so.
The main difference is that the newer lenses compensate for more aspects. Chromatic aberration and corner sharpness being the most obvious ones, but potentially also vignetting (there has also been a trend for new lenses to ignore vignetting with the “fix it in post”-mentality).
I don’t have any experience with the Leica lens, but I’ve got the Canon EF-50mm 1.2 (and 1.4) and both of them are very soft in the corners wide open and they have plenty of chromatic aberrations, which is a consequence of the compact double Gauss optical formula. The Sigma Art 50mm and the RF 50mm 1.2 has a completely different level of corner sharpness and CA.