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 guess it’s something with the quality of the glass. Modern lenses are dead sharp and have very little flaring, ghosting, chromatic aberrations and low distortion. They also focus instantly with zero noise.
Say there’s a short Nikon 35mm f2 which has very bad ghosting situation, and it’s resolution is miles behind modern 35mm f1.8 mirrorless.
Read about elements that go in a lens, you’ll get some idea
Isn’t that leica lens for rangefinder?
Just like any form of miniaturization, making smaller lenses requires tighter tolerances to get similar quality. Big lenses gather more light, reduce vignetting, and more easily allow for larger apertures. When you try to make these things smaller, it requires better materials, more difficult manufacturing, and more complicated engineering. Professional lenses of any brand are expensive. Leica lenses are very, very expensive in part due to these challenges.
Did you just compare a manual lens to one with auto focus?