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.
Well I don’t claim to know everything (or anything really) about lens design but I’ll say this: Years ago they didn’t try to make all these lenses as f1.2 . This is a real sore spot for me especially lens for mirrorless. They make these nice light camera bodies and try to shove a 6lb lens down my throat. I understand that ppl doing Astro work might need/want that but I’d like some lenses under 200grams to go with the body. I’m a Nikon user for over 50 years and damn near ready to ditch them as they are a particularly bad offender