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.
I’ll just add there are great small modern lenses, e.g. Samyang’s tiny lenses for Sony full frame. Not pictured is the 24/1.8 which is a similar size yet similar quality to the Sony GM 24/1.4, other than the aperture, though not all of them are quite to that level optically.
Look at Fujinon 56mm f:1.2. Which is also large.
Yet TTartisan 50mm f:1.2 is really small.
Autofocus means the lens will be larger, for example.
And what is the image quality and do they cover the same sensor size?
Lenses with larger apertures ( the lower the f number the larger the aperture ) let in more light. That requires larger diameter elements. Notice the front element is larger on a 1.2 than say a 1.7. More glass/more weight.
The Nikons have two autofocus motors.