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.
Newer lenses need to somehow improve on their predecessors/competitors to make them marketable, and the trend has been on improving their optical performance (sharpness, aberrations, etc).
In the past, aberrations and lack of sharpness was embraced as “character” for lenses, or at least accepted tradeoffs for ultra-large aperture lenses. Heck, intentionally soft images were in vogue for a while. However, the current trend in the digital age has been toward edge-to-edge optical perfection wide open, with secondary consideration for things like bokeh quality, autofocus speed, handling. With that said, it has always been Leica-M’s aesthetic to feature small lenses to go with their small cameras, and so they are willing to sacrifice some optical performance for that.
And finally, things likely have been helped by modern technology. I’m sure that having access to more computational power since the 70s has made it easier to create these more complex optical designs.